c-family: implement -ffp-contract=on

c-family: implement -ffp-contract=on

Implement -ffp-contract=on for C and C++ without changing default
behavior (=off for -std=cNN, =fast for C++ and -std=gnuNN).

gcc/c-family/ChangeLog:

	* c-gimplify.cc (fma_supported_p): New helper.
	(c_gimplify_expr) [PLUS_EXPR, MINUS_EXPR]: Implement FMA
	contraction.

gcc/ChangeLog:

	* common.opt (fp_contract_mode) [on]: Remove fallback.
	* config/sh/sh.md (*fmasf4): Correct flag_fp_contract_mode test.
	* doc/invoke.texi (-ffp-contract): Update.
	* trans-mem.cc (diagnose_tm_1): Skip internal function calls.