Gracefully handle lua_error() for a suspended coroutine.

Thanks to Sergei Zhirikov.
Merge branch 'master' into v2.1
Cleanup of frame handling. No functional changes.
Avoid dependency on frame layout for LJ_POST_FIXCOMP.

The auxiliary value in u32.lo wasn't used, anyway.
Disable debug info for generated *.S to avoid confusing gdb.
Merge branch 'v2.1' into v2.1-agentzh