ASM support for sparc64 systems

[cjeker]

Add back proper ASM for sparc64 systems that works.

This is needed to support systems without ucontext support. Also the direct ASM is probably a lot faster then the ucontext code
since it avoids extra system calls.

It would be nice if PR #251 would be reviewed.

Also php/php-src#13382 depends on this.

[olk]

done - ty