cmd/ld: abort if (32-bit) address relocation is negative on amd64.

Update #7980
This CL make the linker abort for the example program. For Go 1.4,
we need to find a general way to handle large memory model programs.

LGTM=dave, josharian, iant
R=iant, dave, josharian
CC=golang-codereviews
https://golang.org/cl/91500046

src/cmd/ld/data.c

@@ -243,6 +243,16 @@ relocsym(LSym *s)
 				break;
 			}
 			o = symaddr(r->sym) + r->add;
+
+			// On amd64, 4-byte offsets will be sign-extended, so it is impossible to
+			// access more than 2GB of static data; fail at link time is better than
+			// fail at runtime. See http://golang.org/issue/7980.
+			// Instead of special casing only amd64, we treat this as an error on all
+			// 64-bit architectures so as to be future-proof.
+			if((int32)o < 0 && PtrSize > 4 && siz == 4) {
+				diag("non-pc-relative relocation address is too big: %#llux", o);
+				errorexit();
+			}
 			break;
 		case R_CALL:
 		case R_PCREL: