Fix various clang compile issues

Fix various clang compile issues

1) remove uses of __label__, which is not supported by llvm/clang
2) remove uses of register type var __asm("ecx") and other variable
register-binding as it is not supported by llvm/clang and is superfluous
3) add an ugly hack, conditionalized on __clang__, to allow correct
compilation of atomic_intr_cond_try()

Submitted-by: Alex Hornung
Cherry-Picked-From: Alex Hornung's leaf repo