x86: <asm/asm.h> consistency cleanups

Rename _ASM_MOV_UL to _ASM_MOV for consistency with other _ASM_
instructions (_ASM_ADD, _ASM_SUB and so on.)

Add ASM_SP, _ASM_BP, _ASM_SI, and _ASM_DI for consistency with
_ASM_[ABCD]X.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>