rs6000: Don't use rs6000_isa_flags_explicit for soft float tests (PR88145)

rs6000: Don't use rs6000_isa_flags_explicit for soft float tests (PR88145)

...specifically, those for builtins.  Soft float can be enabled
implicitly, too (for certain CPUs for example).  We should use
rs6000_isa_flags instead, to decide whether to expand a builtin or
to bail out with an error instead.

	PR target/88145
	* config/rs6000/rs6000.c (rs6000_expand_zeroop_builtin): Use
	rs6000_isa_flags instead of rs6000_isa_flags_explicit to decide
	whether soft float is enabled.
	(rs6000_expand_mtfsb_builtin): Ditto.
	(rs6000_expand_set_fpscr_rn_builtin): Ditto.
	(rs6000_expand_set_fpscr_drn_builtin): Ditto.

From-SVN: r266973