x86: Simplify syscallhdr.sh

Simplify syscallhdr.sh by letting grep sort out the ABIs that we want,
rather than relying on manual list matching.  This is safe since the
ABI strings already have to consist only of characters which are valid in C
macro names.

Suggested-by: Matt Helsley <matthltc@us.ibm.com>
Link: http://lkml.kernel.org/r/20111118221558.GA6408@count0.beaverton.ibm.com


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