cmd/internal/obj: sort relocations

At least on some platforms (e.g. PE dynamic loader) relocations
need to be sorted in address order. Currently we don't always emit
relocations in address order: e.g. for array literal with out-of-
order element initializers, or out-of-order DATA instructions in
assembly code. Sort them.

No test for now as I can't reproduce the failure for golang#51923.

Fixes golang#51923.

Change-Id: Ifec5d3476e027bb927bcefd6e45c40ebeccee4ba
Reviewed-on: https://go-review.googlesource.com/c/go/+/396195
Trust: Cherry Mui <cherryyz@google.com>
Run-TryBot: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>

src/cmd/internal/obj/objfile.go

@@ -173,6 +173,7 @@ func WriteObjFile(ctxt *Link, b *bio.Writer) {
 	h.Offsets[goobj.BlkReloc] = w.Offset()
 	for _, list := range lists {
 		for _, s := range list {
+			sort.Sort(relocByOff(s.R)) // some platforms (e.g. PE) requires relocations in address order
 			for i := range s.R {
 				w.Reloc(&s.R[i])
 			}