LoongArch: testsuite:Give up the detection of the gcc.dg/fma-{3, 4, 6, 7}.c file.

On the LoongArch architecture, the above four test cases need to be waived
during testing. There are two situations:

1. The function of fma-{3,6}.c test is to find the value of c-a*b, but on
the LoongArch architecture, the function of the existing fnmsub instruction
is to find the value of -(a*b - c);

2. The function of fma-{4,7}.c test is to find the value of -(a*b)-c, but on
the LoongArch architecture, the function of the existing fnmadd instruction
is to find the value of -(a*b + c);

Through the analysis of the above two cases, there will be positive and
negative zero inequality.

gcc/testsuite/ChangeLog

	* gcc.dg/fma-3.c: The intermediate file corresponding to the
	function does not produce the corresponding FNMA symbol, so the test
	rules should be skipped when testing.
	* gcc.dg/fma-4.c: The intermediate file corresponding to the
	function does not produce the corresponding FNMS symbol, so skip the
	test rules when testing.
	* gcc.dg/fma-6.c: The cause is the same as fma-3.c.
	* gcc.dg/fma-7.c: The cause is the same as fma-4.c