(-a)*b should not be compiled to vnmul a,b with -frounding-math.
Added a new -(a*b) pattern for vnmul and the old one is only
used if !flag_rounding_math.  Updated the costs too.

This is the ARM version of
https://gcc.gnu.org/ml/gcc-patches/2015-07/msg00300.html

Tested with arm-none-linux-gnueabihf cross compiler.
is this OK?

gcc/Changelog:

2015-07-20  Szabolcs Nagy  <szabolcs.nagy@arm.com>

	PR target/66731
	* config/arm/arm.md (muldf3negdf_vfp): Handle -frounding-math.
	(mulsf3negsf_vfp): Likewise.
	* config/arm/arm.c (arm_new_rtx_costs): Fix NEG cost for VNMUL,
	fix MULT cost with -frounding-math.

gcc/testsuite/Changelog:

2015-07-20  Szabolcs Nagy  <szabolcs.nagy@arm.com>

	PR target/66731
	* gcc.target/arm/vnmul-1.c: New.
	* gcc.target/arm/vnmul-2.c: New.
	* gcc.target/arm/vnmul-3.c: New.
	* gcc.target/arm/vnmul-4.c: New.