[Driver] Corrections for linker flags passed with relocatable linking on OpenBSD (#67254)

The entry point symbol handling matches our GCC link spec..
```%{!shared:%{!nostdlib:%{!r:%{!e*:-e __start}}}}```

Remove usage of -Bdynamic as it is the default for the linker anyway.

Came up in discussion here #65644

Author: brad0

clang/lib/Driver/ToolChains/OpenBSD.cpp

@@ -121,6 +121,7 @@ void openbsd::Linker::ConstructJob(Compilation &C, const JobAction &JA,
   bool Profiling = Args.hasArg(options::OPT_pg);
   bool Pie = Args.hasArg(options::OPT_pie);
   bool Nopie = Args.hasArg(options::OPT_nopie);
+  const bool Relocatable = Args.hasArg(options::OPT_r);
 
   // Silence warning for "clang -g foo.o -o foo"
   Args.ClaimAllArgs(options::OPT_g_Group);
@@ -138,7 +139,7 @@ void openbsd::Linker::ConstructJob(Compilation &C, const JobAction &JA,
   else if (Arch == llvm::Triple::mips64el)
     CmdArgs.push_back("-EL");
 
-  if (!Args.hasArg(options::OPT_nostdlib) && !Shared) {
+  if (!Args.hasArg(options::OPT_nostdlib) && !Shared && !Relocatable) {
     CmdArgs.push_back("-e");
     CmdArgs.push_back("__start");
   }
@@ -149,10 +150,9 @@ void openbsd::Linker::ConstructJob(Compilation &C, const JobAction &JA,
   } else {
     if (Args.hasArg(options::OPT_rdynamic))
       CmdArgs.push_back("-export-dynamic");
-    CmdArgs.push_back("-Bdynamic");
     if (Shared) {
       CmdArgs.push_back("-shared");
-    } else if (!Args.hasArg(options::OPT_r)) {
+    } else if (!Relocatable) {
       CmdArgs.push_back("-dynamic-linker");
       CmdArgs.push_back("/usr/libexec/ld.so");
     }

clang/test/Driver/openbsd.c

@@ -8,7 +8,7 @@
 // RUN: %clang --target=i686-pc-openbsd -pg -pthread -### %s 2>&1 \
 // RUN:   | FileCheck --check-prefix=CHECK-PG %s
 // CHECK-PG: "-cc1" "-triple" "i686-pc-openbsd"
-// CHECK-PG: ld{{.*}}" "-e" "__start" "--eh-frame-hdr" "-Bdynamic" "-dynamic-linker" "{{.*}}ld.so" "-nopie" "-o" "a.out" "{{.*}}gcrt0.o" "{{.*}}crtbegin.o" "{{.*}}.o" "-lcompiler_rt" "-lpthread_p" "-lc_p" "-lcompiler_rt" "{{.*}}crtend.o"
+// CHECK-PG: ld{{.*}}" "-e" "__start" "--eh-frame-hdr" "-dynamic-linker" "{{.*}}ld.so" "-nopie" "-o" "a.out" "{{.*}}gcrt0.o" "{{.*}}crtbegin.o" "{{.*}}.o" "-lcompiler_rt" "-lpthread_p" "-lc_p" "-lcompiler_rt" "{{.*}}crtend.o"
 
 // Check CPU type for i386
 // RUN: %clang --target=i386-unknown-openbsd -### -c %s 2>&1 \
@@ -34,18 +34,19 @@
 // RUN:   | FileCheck --check-prefix=CHECK-MIPS64-LD %s
 // RUN: %clang --target=mips64el-unknown-openbsd -### %s 2>&1 \
 // RUN:   | FileCheck --check-prefix=CHECK-MIPS64EL-LD %s
-// CHECK-LD-R:     "-r"
+// CHECK-LD-R-NOT: "-e"
 // CHECK-LD-R-NOT: "-dynamic-linker"
 // CHECK-LD-R-NOT: "-l
 // CHECK-LD-R-NOT: crt{{[^./\\]+}}.o
+// CHECK-LD-R:     "-r"
 // CHECK-LD-S: "-cc1" "-triple" "i686-pc-openbsd"
-// CHECK-LD-S: ld{{.*}}" "-e" "__start" "--eh-frame-hdr" "-Bdynamic" "-dynamic-linker" "{{.*}}ld.so" "-o" "a.out" "{{.*}}crt0.o" "{{.*}}crtbegin.o" "-L{{.*}}" "-s" "{{.*}}.o" "-lcompiler_rt" "-lc" "-lcompiler_rt" "{{.*}}crtend.o"
+// CHECK-LD-S: ld{{.*}}" "-e" "__start" "--eh-frame-hdr" "-dynamic-linker" "{{.*}}ld.so" "-o" "a.out" "{{.*}}crt0.o" "{{.*}}crtbegin.o" "-L{{.*}}" "-s" "{{.*}}.o" "-lcompiler_rt" "-lc" "-lcompiler_rt" "{{.*}}crtend.o"
 // CHECK-LD-T: "-cc1" "-triple" "i686-pc-openbsd"
-// CHECK-LD-T: ld{{.*}}" "-e" "__start" "--eh-frame-hdr" "-Bdynamic" "-dynamic-linker" "{{.*}}ld.so" "-o" "a.out" "{{.*}}crt0.o" "{{.*}}crtbegin.o" "-L{{.*}}" "-t" "{{.*}}.o" "-lcompiler_rt" "-lc" "-lcompiler_rt" "{{.*}}crtend.o"
+// CHECK-LD-T: ld{{.*}}" "-e" "__start" "--eh-frame-hdr" "-dynamic-linker" "{{.*}}ld.so" "-o" "a.out" "{{.*}}crt0.o" "{{.*}}crtbegin.o" "-L{{.*}}" "-t" "{{.*}}.o" "-lcompiler_rt" "-lc" "-lcompiler_rt" "{{.*}}crtend.o"
 // CHECK-MIPS64-LD: "-cc1" "-triple" "mips64-unknown-openbsd"
-// CHECK-MIPS64-LD: ld{{.*}}" "-EB" "-e" "__start" "--eh-frame-hdr" "-Bdynamic" "-dynamic-linker" "{{.*}}ld.so" "-o" "a.out" "{{.*}}crt0.o" "{{.*}}crtbegin.o" "-L{{.*}}" "{{.*}}.o" "-lcompiler_rt" "-lc" "-lcompiler_rt" "{{.*}}crtend.o"
+// CHECK-MIPS64-LD: ld{{.*}}" "-EB" "-e" "__start" "--eh-frame-hdr" "-dynamic-linker" "{{.*}}ld.so" "-o" "a.out" "{{.*}}crt0.o" "{{.*}}crtbegin.o" "-L{{.*}}" "{{.*}}.o" "-lcompiler_rt" "-lc" "-lcompiler_rt" "{{.*}}crtend.o"
 // CHECK-MIPS64EL-LD: "-cc1" "-triple" "mips64el-unknown-openbsd"
-// CHECK-MIPS64EL-LD: ld{{.*}}" "-EL" "-e" "__start" "--eh-frame-hdr" "-Bdynamic" "-dynamic-linker" "{{.*}}ld.so" "-o" "a.out" "{{.*}}crt0.o" "{{.*}}crtbegin.o" "-L{{.*}}" "{{.*}}.o" "-lcompiler_rt" "-lc" "-lcompiler_rt" "{{.*}}crtend.o"
+// CHECK-MIPS64EL-LD: ld{{.*}}" "-EL" "-e" "__start" "--eh-frame-hdr" "-dynamic-linker" "{{.*}}ld.so" "-o" "a.out" "{{.*}}crt0.o" "{{.*}}crtbegin.o" "-L{{.*}}" "{{.*}}.o" "-lcompiler_rt" "-lc" "-lcompiler_rt" "{{.*}}crtend.o"
 
 // Check that --sysroot is passed to the linker
 // RUN: %clang --target=i686-pc-openbsd -### %s 2>&1 \