[LLD] Add support for .openbsd.syscalls

(rebaser's note) adapted from:
openbsd/src@42a61ac

Collect .openbsd.syscalls sections into a new PT_OPENBSD_SYSCALLS segment. This will be used soon to pin system calls to designated call sites.

ok deraadt@

Author: kettenis

lld/ELF/ScriptParser.cpp

@@ -1630,6 +1630,7 @@ unsigned ScriptParser::readPhdrType() {
                      .Case("PT_GNU_RELRO", PT_GNU_RELRO)
                      .Case("PT_OPENBSD_MUTABLE", PT_OPENBSD_MUTABLE)
                      .Case("PT_OPENBSD_RANDOMIZE", PT_OPENBSD_RANDOMIZE)
+                     .Case("PT_OPENBSD_SYSCALLS", PT_OPENBSD_SYSCALLS)
                      .Case("PT_OPENBSD_WXNEEDED", PT_OPENBSD_WXNEEDED)
                      .Case("PT_OPENBSD_BOOTDATA", PT_OPENBSD_BOOTDATA)
                      .Default(-1);

lld/ELF/Writer.cpp

@@ -2256,6 +2256,11 @@ SmallVector<PhdrEntry *, 0> Writer<ELFT>::createPhdrs(Partition &part) {
   if (OutputSection *cmd = findSection(".openbsd.randomdata", partNo))
     addHdr(PT_OPENBSD_RANDOMIZE, cmd->getPhdrFlags())->add(cmd);
 
+  // PT_OPENBSD_SYSCALLS is an OpenBSD-specific feature. That makes
+  // the kernel and dynamic linker register system call sites.
+  if (OutputSection *cmd = findSection(".openbsd.syscalls", partNo))
+    addHdr(PT_OPENBSD_SYSCALLS, cmd->getPhdrFlags())->add(cmd);
+
   if (config->zGnustack != GnuStackKind::None) {
     // PT_GNU_STACK is a special section to tell the loader to make the
     // pages for the stack non-executable. If you really want an executable