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
