x86: use __ASSEMBLY__ rather than __ASSEMBLER__

As Ingo pointed out in a separate patch, we should be using __ASSEMBLY__.
Make that the case in pgtable headers.

Signed-off-by: Andres Salomon <dilinger@queued.net>
LKML-Reference: <20100605114042.35ac69c1@dev.queued.net>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>