[dev.link] cmd: support large function alignment

This ports CL 226997 to the dev.link branch.
- The assembler part and old object file writing are unchanged.
- Changes to cmd/link are applied to cmd/oldlink.
- Add alignment field to new object files for the new linker.

Change-Id: Id00f323ae5bdd86b2709a702ee28bcaa9ba962f8
Reviewed-on: https://go-review.googlesource.com/c/go/+/227025
Reviewed-by: Than McIntosh <thanm@google.com>

src/cmd/link/internal/loader/loader.go

@@ -1616,6 +1616,9 @@ func (l *Loader) preloadSyms(r *oReader, kind int) {
 			strings.HasPrefix(name, "runtime.gcbits.") {
 			l.SetAttrNotInSymbolTable(gi, true)
 		}
+		if a := osym.Align(); a != 0 {
+			l.SetSymAlign(gi, int32(a))
+		}
 	}
 }