fnmul was modeled as (-a)*b instead of -(a*b), which is wrong with
-frounding-math, so the correct pattern is added too and the other
one is only used if !flag_rounding_math.

This affects a glibc math test, similar fix will be needed for ARM.

Tested with aarch64-none-linux-gnu cross compiler.
is this OK?

gcc/Changelog:

2015-07-06  Szabolcs Nagy  <szabolcs.nagy@arm.com>

	PR target/66731
	* config/aarch64/aarch64.md (fnmul<mode>3): Handle -frounding-math.

gcc/testsuite/Changelog:

2015-07-06  Szabolcs Nagy  <szabolcs.nagy@arm.com>

	* gcc.target/aarch64/fnmul-1.c: New.
	* gcc.target/aarch64/fnmul-2.c: New.
	* gcc.target/aarch64/fnmul-3.c: New.
	* gcc.target/aarch64/fnmul-4.c: New.