Unskip test cases from netbsd-tests by defining __HAVE_FENV

This unskips:

• lib.libc.stdlib.strtod_test.strtod_round
• lib.msun.fe_round_test.t_nofe_round

In lib/msun/tests/Makefile only define on fe_round_test.c because
lib.msun.ilogb_test.ilogb will get wrong results and needs more examination.

MFC after: 1 week
Sponsored by: The FreeBSD Foundation