ld: allow for IMAGE_REL_AMD64_ADDR32NB relocation type

enable cgo again on windows/amd64

R=rsc, vcc.163
CC=golang-dev
https://golang.org/cl/5488074

src/Make.inc

@@ -142,9 +142,6 @@ endif
 ifeq ($(GOOS),netbsd)  # ... and not on NetBSD
 CGO_ENABLED:=0
 endif
-ifeq ($(GOOS)$(GOARCH),windowsamd64)  # ... and not on Windows/amd64
-CGO_ENABLED:=0
-endif
 endif
 
 # Make environment more standard.

src/cmd/ld/ldpe.c

@@ -282,6 +282,8 @@ ldpe(Biobuf *f, char *pkg, int64 len, char *pn)
 					diag("%s: unknown relocation type %d;", pn, type);
 				case IMAGE_REL_I386_REL32:
 				case IMAGE_REL_AMD64_REL32:
+				case IMAGE_REL_AMD64_ADDR32: // R_X86_64_PC32
+				case IMAGE_REL_AMD64_ADDR32NB:
 					rp->type = D_PCREL;
 					rp->add = le32(rsect->base+rp->off);
 					break;
@@ -291,10 +293,6 @@ ldpe(Biobuf *f, char *pkg, int64 len, char *pn)
 					// load addend from image
 					rp->add = le32(rsect->base+rp->off);
 					break;
-				case IMAGE_REL_AMD64_ADDR32: // R_X86_64_PC32
-					rp->type = D_PCREL;
-					rp->add += 4;
-					break;
 				case IMAGE_REL_AMD64_ADDR64: // R_X86_64_64
 					rp->siz = 8;
 					rp->type = D_ADDR;