cmd/ld: do not relocate ignored PE sections

This should have been part of CL8715043.

Update #5106

R=golang-dev, r, iant
CC=golang-dev
https://golang.org/cl/8676050

src/cmd/ld/ldpe.c

@@ -257,6 +257,11 @@ ldpe(Biobuf *f, char *pkg, int64 len, char *pn)
 			continue;
 		if(rsect->sh.Characteristics&IMAGE_SCN_MEM_DISCARDABLE)
 			continue;
+		if((sect->sh.Characteristics&(IMAGE_SCN_CNT_CODE|IMAGE_SCN_CNT_INITIALIZED_DATA|IMAGE_SCN_CNT_UNINITIALIZED_DATA)) == 0) {
+			// This has been seen for .idata sections, which we
+			// want to ignore.  See issues 5106 and 5273.
+			continue;
+		}
 		r = mal(rsect->sh.NumberOfRelocations*sizeof r[0]);
 		Bseek(f, obj->base+rsect->sh.PointerToRelocations, 0);
 		for(j=0; j<rsect->sh.NumberOfRelocations; j++) {