testsuite: libgcc function name for PRU

Update the match rules to accommodate the non-standard libgcc function
names for PRU backend.

gcc/testsuite/ChangeLog:

	* gcc.c-torture/compile/attr-complex-method-2.c: Accept both __divdc3
	and __gnu_divdc3 as valid libgcc function names.
	* gcc.dg/complex-6.c: Ditto for __mulsc3.
	* gcc.dg/complex-7.c: Ditto for __muldc3.

Signed-off-by: Dimitar Dimitrov <dimitar@dinux.eu>