[PATCH] x32: Fix, optimize and cleanup RSEQ_* accessors

[Florian Weimer]

* Uros Bizjak:

> Add missing "memory" clobber to accessors.
>
> Use MOVZBL instead of MOVB to read 1-byte memory location to
> a register.
>
> Change %P asm operand modifiers to %c.
>
> Add explicit casts instead of using %b and %q asm operand modifiers.

Please use the commit message to explain the reasons behind these
changes.  The changes themselves are evident from the diff.

If you use movzbl, should you change the type of __value to long long
int, to hint to the compiler that the entire register is cleared?