lib/msun/tests: Skip fenv_test:masking if exceptions can't be trapped

[arichardson]

Summary

Some CPUs (e.g. AArch64 QEMU) cannot trap on floating point exceptions and
therefore ignore the writes to the floating point control register inside
feenableexcept(). If no exceptions are enabled after
feenableexcept(FE_ALL_EXCEPT), we can assume that the CPU does not
support exceptions and we can then skip the test.

[dim]

Looks like a good idea to me.