Accept at most a single constant for fma patterns.

gcc/
2018-03-21  Claudiu Zissulescu  <claziss@synopsys.com>

	* config/arc/fpu.md (fmasf4): Force operand to register.
	(fnmasf4): Likewise.

gcc/testsuite
2018-03-21  Claudiu Zissulescu  <claziss@synopsys.com>

	* gcc.target/arc/fma-1.c: New test.

From-SVN: r261543