x86, realmode: Mask out EFER.LMA when saving trampoline EFER

Some AMD processors apparently #GP(0) if EFER.LMA is set in WRMSR,
rather than ignoring it.  Thus, we need to mask it out.

Reported-by: Ingo Molnar <mingo@kernel.org>
Tested-by: Borislav Petkov <bp@alien8.de>
Cc: Jarkko Sakkinen <jarkko.sakkinen@intel.com>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>
Link: http://lkml.kernel.org/r/1336501366-28617-24-git-send-email-jarkko.sakkinen@intel.com