cmd/link: use internal linking for -race mode on darwin/arm64

The code I wrote in ldmacho.go in CL 266373 was plainly wrong. It
didn't carry rAdd over correctly. Fixed. Also added sign extension
(as ld64 does).

Internal linking with -race mode now works. Enable it.

Updates #38485.

Change-Id: I78aa949687bf6a0987913059059160b018c7560e
Reviewed-on: https://go-review.googlesource.com/c/go/+/267097
Trust: Cherry Zhang <[email protected]>
Reviewed-by: Than McIntosh <[email protected]>

Author: cherrymui

src/cmd/link/internal/ld/config.go

@@ -206,7 +206,7 @@ func mustLinkExternal(ctxt *Link) (res bool, reason string) {
 	// When the race flag is set, the LLVM tsan relocatable file is linked
 	// into the final binary, which means external linking is required because
 	// internal linking does not support it.
-	if *flagRace && (ctxt.Arch.InFamily(sys.PPC64) || ctxt.IsDarwin() && ctxt.IsARM64()) {
+	if *flagRace && ctxt.Arch.InFamily(sys.PPC64) {
 		return true, "race on " + objabi.GOARCH
 	}
 

src/cmd/link/internal/loadmacho/ldmacho.go

@@ -724,10 +724,8 @@ func Load(l *loader.Loader, arch *sys.Arch, localSymVersion int, f *bio.Reader,
 
 			if arch.Family == sys.ARM64 && rel.type_ == MACHO_ARM64_RELOC_ADDEND {
 				// Two relocations. This addend will be applied to the next one.
-				rAdd = int64(rel.symnum)
+				rAdd = int64(rel.symnum) << 40 >> 40 // convert unsigned 24-bit to signed 24-bit
 				continue
-			} else {
-				rAdd = 0
 			}
 
 			rSize = rel.length
@@ -789,6 +787,8 @@ func Load(l *loader.Loader, arch *sys.Arch, localSymVersion int, f *bio.Reader,
 			r.SetSiz(rSize)
 			r.SetSym(rSym)
 			r.SetAdd(rAdd)
+
+			rAdd = 0 // clear rAdd for next iteration
 		}
 
 		sb.SortRelocs()