ldxr_stxr does an ldxr on thread 0 to fall in the ISA case where event is certain.
Also add ldxr_str which tests yet another case where event is certain: non conditional
write.
32-bit ones likely copy paste error from coming from arm v7.
Also create userland/arch/aarch64/inline_asm/int_32.c:
Also create aarch64_ldaxr_stlxr.cpp and start documenting LDAXR and STLXR.