PR libstdc++/78179 run long double tests separately

PR libstdc++/78179 run long double tests separately

Split the long double testing into a separate file, so that we can XFAIL
targets where the long double precision doesn't meet the expected
tolerances. The float and double tests are still expefted to PASS for
all targets.

	PR libstdc++/78179
	* testsuite/26_numerics/headers/cmath/hypot-long-double.cc: New test
	that runs the long double part of hypot.cc.
	* testsuite/26_numerics/headers/cmath/hypot.cc: Disable long double
	tests unless TEST_HYPOT_LONG_DOUBLE is defined.

From-SVN: r264063