powerpc: Fix VSCR position in ucontext (bug 24088)

This patch fix VSCR position on ucontext. VSCR was read in the wrong
position on ucontext structure because it was ignoring the machine
endianess.

	[BZ #24088]
	* sysdeps/unix/sysv/linux/powerpc/sys/ucontext.h (vscr_t): Added
	ifdef to fix read of VSCR.
	* sysdeps/powerpc/powerpc64/Makefile [$subdir == stdlib]: Add
	tst-ucontext-ppc64-vscr.c to test list.
	* sysdeps/powerpc/powerpc64/tst-ucontext-ppc64-vscr.c: New test file.

Reviewed-by: Tulio Magno Quites Machado Filho <tuliom@linux.ibm.com>