[PATCH] power: Fix VSCR position on ucontext

[Joseph Myers]

I'm seeing the test fail to build for powerpc64-linux-gnu (big-endian) 
with my build-many-glibcs.py bots.  (That's at least with GCC 7 and GCC 8; 
my GCC mainline bot hasn't yet run after that commit.)

../sysdeps/powerpc/powerpc64/tst-ucontext-ppc64-vscr.c: In function 'do_test':
../sysdeps/powerpc/powerpc64/tst-ucontext-ppc64-vscr.c:51:3: error: inconsistent operand constraints in an 'asm'
   asm volatile (".machine push;\n"
   ^~~
/scratch/jmyers/glibc-bot/build/glibcs/powerpc64-linux-gnu/glibc/sysd-rules:933: recipe for target '/scratch/jmyers/glibc-bot/build/glibcs/powerpc64-linux-gnu/glibc/stdlib/tst-ucontext-ppc64-vscr.o' failed

-- 
Joseph S. Myers
joseph@codesourcery.com