[gn] port c253e5c (RISCV llvm-exegesis)

See here for the additional tblgen deps:
#128767 (comment)

Author: nico

llvm/utils/gn/secondary/llvm/lib/Target/RISCV/BUILD.gn

@@ -17,6 +17,7 @@ tablegen("RISCVGenCompressInstEmitter") {
 tablegen("RISCVGenMacroFusion") {
   visibility = [
     ":LLVMRISCVCodeGen",
+    "//llvm/tools/llvm-exegesis/lib/RISCV",
     "//llvm/unittests/Target/RISCV:RISCVTests",
   ]
   args = [ "-gen-macro-fusion-pred" ]
@@ -69,7 +70,10 @@ tablegen("RISCVGenPostLegalizeGICombiner") {
 }
 
 tablegen("RISCVGenRegisterBank") {
-  visibility = [ ":LLVMRISCVCodeGen" ]
+  visibility = [
+    ":LLVMRISCVCodeGen",
+    "//llvm/tools/llvm-exegesis/lib/RISCV",
+  ]
   args = [ "-gen-register-bank" ]
   td_file = "RISCV.td"
 }

llvm/utils/gn/secondary/llvm/tools/llvm-exegesis/lib/RISCV/BUILD.gn

@@ -15,8 +15,14 @@ static_library("RISCV") {
 
     # Exegesis reaches inside the Target/RISCV tablegen internals and must
     # depend on these Target/RISCV-internal build targets.
+    "//llvm/lib/Target/RISCV:RISCVGenMacroFusion",
+    "//llvm/lib/Target/RISCV:RISCVGenRegisterBank",
     "//llvm/lib/Target/RISCV/MCTargetDesc",
   ]
-  sources = [ "Target.cpp" ]
+  sources = [
+    "RISCVExegesisPostprocessing.cpp",
+    "RISCVExegesisPreprocessing.cpp",
+    "Target.cpp",
+  ]
   include_dirs = [ "//llvm/lib/Target/RISCV" ]
 }