cmd/ld: cast PE32 absolute addend to int32.

Didn't manage to find a way to write test cases.

Fixes #7769.

LGTM=iant
R=iant
CC=golang-codereviews
https://golang.org/cl/88000045

src/cmd/ld/ldpe.c

@@ -297,7 +297,7 @@ ldpe(Biobuf *f, char *pkg, int64 len, char *pn)
 				case IMAGE_REL_I386_DIR32:
 					rp->type = R_ADDR;
 					// load addend from image
-					rp->add = le32(rsect->base+rp->off);
+					rp->add = (int32)le32(rsect->base+rp->off);
 					break;
 				case IMAGE_REL_AMD64_ADDR64: // R_X86_64_64
 					rp->siz = 8;