rs6000: Guard __builtin_{un,}pack_vector_int128 with vsx [PR109932]

As PR109932 shows, builtins __builtin_{un,}pack_vector_int128
should be guarded under vsx rather than power7, as their
corresponding bif patterns have the conditions TARGET_VSX
and VECTOR_MEM_ALTIVEC_OR_VSX_P (V1TImode).  This patch is to
move __builtin_{un,}pack_vector_int128 to stanza vsx to ensure
their supports.

	PR target/109932

gcc/ChangeLog:

	* config/rs6000/rs6000-builtins.def (__builtin_pack_vector_int128,
	__builtin_unpack_vector_int128): Move from stanza power7 to vsx.

gcc/testsuite/ChangeLog:

	* gcc.target/powerpc/pr109932-1.c: New test.
	* gcc.target/powerpc/pr109932-2.c: New test.