sys/sys: Check for __ASSEMBLER__ in elf_common.h

[andrew]

Summary

When preprocessing assembly files with clang or gcc the ASSEMBLER
macro is defined. Check for this as an alternative to LOCORE in
elf_common.h so it can be included by .S files.

Sponsored by: Arm Ltd

[imp]

This is fine...

But maybe we don't need the LOCORE hack at all if all assemblers define ASSEMBLER now?

[andrew]

I don't think we need LOCORE. I can follow up with a commit to remove LOCORE, although I'm unsure if we have anything that depends on it (either base or ports)