[X86] Remove MWAITX_SAVE_EBX pseudo instruction. Always save/restore the full...

[X86] Remove MWAITX_SAVE_EBX pseudo instruction. Always save/restore the full %rbx register even in gnux32.

ebx/rbx only needs to be saved when 64-bit registers are supported
anyway. It should be fine to save/restore the whole rbx register
even in gnux32 where the base is technically just ebx.

This matches what we do for cmpxchg16b where rbx is saved/restored
regardless of gnux32.