cmd/internal/ld: put all type decoding in decodesym.go

Move the one instance of type structure decoding in the linker that
doesn't live decodesym.go in to decodesym.go.

Change-Id: Ic6a23500deb72f0e9c8227ab611511e9781fac70
Reviewed-on: https://go-review.googlesource.com/9690Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/internal/ld/decodesym.go

@@ -75,6 +75,10 @@ func decodetype_gcprog(s *LSym) *LSym {
 	return decode_reloc_sym(s, 1*int32(Thearch.Ptrsize)+8+2*int32(Thearch.Ptrsize))
 }
 
+func decodetype_gcprog_shlib(s *LSym) uint64 {
+	return decode_inuxi(s.P[1*int32(Thearch.Ptrsize)+8+1*int32(Thearch.Ptrsize):], Thearch.Ptrsize)
+}
+
 func decodetype_gcmask(s *LSym) []byte {
 	if s.Type == obj.SDYNIMPORT {
 		// ldshlibsyms makes special efforts to read the value

src/cmd/internal/ld/lib.go

@@ -1224,10 +1224,7 @@ func ldshlibsyms(shlib string) {
 		if decodetype_noptr(t) != 0 || decodetype_usegcprog(t) != 0 {
 			continue
 		}
-		// The expression on the next line is a copy of the expression from
-		// decodetype_gcmask in decodesym.go, which in turn depends on details of
-		// how the type data is laid out, as seen in gc/reflect.go:dcommontype.
-		addr := decode_inuxi(t.P[1*int32(Thearch.Ptrsize)+8+1*int32(Thearch.Ptrsize):], Thearch.Ptrsize)
+		addr := decodetype_gcprog_shlib(t)
 		tgcmask, ok := gcmasks[addr]
 		if !ok {
 			Diag("bits not found for %s at %d", t.Name, addr)