[ARC] Don't use drsub* instructions when selecting fpuda.

[ARC] Don't use drsub* instructions when selecting fpuda.

The double precision floating point assist instructions are not
implementing the reverse double subtract instruction (drsub) found in
the FPX extension.

gcc/
2016-04-28  Claudiu Zissulescu  <claziss@synopsys.com>

	* config/arc/arc.md (cpu_facility): Add fpx variant.
	(subdf3): Prohibit use reverse sub when assist operations option
	is enabled.
	* config/arc/fpx.md (subdf3_insn, *dsubh_peep2_insn): Allow drsub
	instructions only when FPX is enabled.
        * testsuite/gcc.target/arc/trsub.c: New test.

From-SVN: r235562