cmd/link: implement R_X86_64_PC64 relocations

Change-Id: I1d7bd5cff7350a4e0f78b8efc8406e79c74732d1
Reviewed-on: https://go-review.googlesource.com/55370Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/link/internal/amd64/asm.go

@@ -118,6 +118,17 @@ func adddynrel(ctxt *ld.Link, s *ld.Symbol, r *ld.Reloc) bool {
 		r.Add += 4
 		return true
 
+	case 256 + ld.R_X86_64_PC64:
+		if targ.Type == ld.SDYNIMPORT {
+			ld.Errorf(s, "unexpected R_X86_64_PC64 relocation for dynamic symbol %s", targ.Name)
+		}
+		if targ.Type == 0 || targ.Type == ld.SXREF {
+			ld.Errorf(s, "unknown symbol %s in pcrel", targ.Name)
+		}
+		r.Type = objabi.R_PCREL
+		r.Add += 8
+		return true
+
 	case 256 + ld.R_X86_64_PLT32:
 		r.Type = objabi.R_PCREL
 		r.Add += 4

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

@@ -1210,6 +1210,7 @@ func relSize(ctxt *Link, pn string, elftype uint32) uint8 {
 		return 4
 
 	case AMD64 | R_X86_64_64<<24,
+		AMD64 | R_X86_64_PC64<<24,
 		PPC64 | R_PPC64_ADDR64<<24,
 		S390X | R_390_GLOB_DAT<<24,
 		S390X | R_390_RELATIVE<<24,