Testsuite: Mark check_effective_target_exceptions_enabled test as C++ test input.

Testsuite: Mark check_effective_target_exceptions_enabled test as C++ test input.

The test in check_effective_target_exceptions_enabled uses a C++ keyword `throw`
and the test fails with a syntax error on any non-g++ test.  I now tell the
testsuite driver that this is a C++ input file so it runs it as such in all the
drivers.

gcc/testsuite/ChangeLog:

	* lib/target-supports.exp (check_effective_target_exceptions_enabled):
	Mark as C++ test input.