x86, xsave: add <asm/xcr.h> header file for XCR registers

Add <asm-x86/xcr.h> header file for the XCR registers and their access
functions.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>