Fixes incorrect mask being used when FE_INVALID bit is wanted by user.

The problem was noticed thanks lib msun fenv tests, that was failing on test 3.

*fenv test still crashes on test 6, this will be addressed on another review

Tested on powerpc64. Without the patch When running fenv_test, it should print "ok 3 - fenv". It will crash with assert after "ok 2 - fenv" when unpatched.

$ ./fenv_test
1..8
ok 1 - fenv
ok 2 - fenv
ok 3 - fenv
ok 4 - fenv
ok 5 - fenv
Assertion failed: (fetestexcept(FE_ALL_EXCEPT) == excepts), function test_fegsetenv, file /usr/home/alfredo/src/freebsd/lib/msun/tests/fenv_test.c, line 376.
Abort trap (core dumped)