x86-64, asm: If the assembler supports fxsave64, use it

x86-64, asm: If the assembler supports fxsave64, use it

Kbuild allows for us to probe for the existence of specific constructs
in the assembler, use them to find out if we can use fxsave64 and
permit the compiler to generate better code.

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