rs6000: Guard power9-vector for vsx_scalar_cmp_exp_qp_* [PR108758]

__builtin_vsx_scalar_cmp_exp_qp_{eq,gt,lt,unordered} used
to be guarded with condition TARGET_P9_VECTOR before new
bif framework was introduced (r12-5752-gd08236359eb229),
since r12-5752 they are placed under stanza ieee128-hw,
that is to check condition TARGET_FLOAT128_HW, it caused
test case float128-cmp2-runnable.c to fail at -m32 as the
condition TARGET_FLOAT128_HW isn't satisified with -m32.

By checking the commit history, I didn't see any notes on
why this condition change on them was made, so this patch
is to move these bifs from stanza ieee128-hw to stanza
power9-vector as before.

	PR target/108758

gcc/ChangeLog:

	* config/rs6000/rs6000-builtins.def
	(__builtin_vsx_scalar_cmp_exp_qp_eq, __builtin_vsx_scalar_cmp_exp_qp_gt
	__builtin_vsx_scalar_cmp_exp_qp_lt,
	__builtin_vsx_scalar_cmp_exp_qp_unordered): Move from stanza ieee128-hw
	to power9-vector.