std: Explicitly link to libm for freebsd

Apparently we had forgotten to do this for freebsd, causing possible problems
on FreeBSD 10. The discussion in #12324 has some more details about how it's
missing.
auto merge of #13134 : alexcrichton/rust/freebsd-libm, r=thestinger

Apparently we had forgotten to do this for freebsd, causing possible problems
on FreeBSD 10. The discussion in #12324 has some more details about how it's
missing.