[StackMaps] Be explicit about label formation [NFC] (try 2)

Recommit after making the same API change in non-x86 targets.  This has been build for all targets, and tested for effected ones.  Why the difference?  Because my disk filled up when I tried make check for all.

For auto-padding assembler support, we'll need to bundle the label with the instructions (nops or call sequences) so that they don't get separated.  This just rearranges the code to make the upcoming change more obvious.