selftests/x86/sigreturn: Do minor cleanups

We have short names for the requested and resulting register values.
Use them instead of spelling out the whole register entry for each
case.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/bb3bc1f923a2f6fe7912d22a1068fe29d6033d38.1530076529.git.luto@kernel.org


Signed-off-by: Ingo Molnar <mingo@kernel.org>