powerpc: fix tst-ucontext-ppc64-vscr test for POWER 5/6.

An error "impossible register constraint in 'asm'" was raised on POWER
5 and due to __vector __int128_t being used as operands without passing the
option -msvx to gcc.
This patch replaces "__vector __int128_t" with "__vector unsigned int"
which requires only -maltivec, available since POWER ISA 2.03, and which
is already passed to the compiler.

	* sysdeps/powerpc/powerpc64/tst-ucontext-ppc64-vscr.c:
	(do_test): Changed __vector __int128_t to __vector unsigned int.

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