cmd/link: fix Macho-O X86_64_RELOC_SUBTRACTOR in internal linking

cmd/link: fix Macho-O X86_64_RELOC_SUBTRACTOR in internal linking

X86_64_RELOC_SUBTRACTOR is handled as a generic R_PCREL relocations,
which gets the relocation size subtracted from the relocated value.

This is not supposed to happen for this particular relocation, so
compensate by adding the size to the addend.

Cq-Include-Trybots: luci.golang.try:gotip-darwin-amd64-race
Change-Id: I6e6889d63bb03b8076e3e409722601dfebec57e5
Reviewed-on: https://go-review.googlesource.com/c/go/+/703776
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>