x86, boot: stylistic cleanups for boot/compressed/head_64.S

Clean up style issues in arch/x86/boot/compressed/head_64.S.  This
file had a lot fewer style issues than its 32-bit cousin, but the ones
it has are worth fixing, especially since it makes the two files more
similar.

[ Impact: cleanup, no object code change ]

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