libc: centralize a few numeric symbols

fabs, infinity, and nan are universally implemented so declare them
in gen/Symbol.map.

We would also include __flt_rounds, but it's under FBSD_1.3 on arm so
until that's gone we're stuck with it. Likewise, everyone but i386
implements fp[gs]etmask.

Reviewed by: imp, kib, emaste
Differential Revision: https://reviews.freebsd.org/D42618

(cherry picked from commit c7045186819dbc64c3f005cb1138a1948868cf5a)