rs6000: Move V2DI vec_neg under power8-vector [PR105271]

rs6000: Move V2DI vec_neg under power8-vector [PR105271]

As PR105271 shows, __builtin_altivec_neg_v2di requires option
-mpower8-vector as its pattern expansion relies on subv2di which
has guard VECTOR_UNIT_P8_VECTOR_P (V2DImode).  This fix is to move
the related lines for __builtin_altivec_neg_v2di to the section
of stanza power8-vector.

	PR target/105271

gcc/ChangeLog:

	* config/rs6000/rs6000-builtins.def (NEG_V2DI): Move to [power8-vector]
	stanza.

gcc/testsuite/ChangeLog:

	* gcc.target/powerpc/pr105271.c: New test.