libc: centralize ntoh symbols

These are implemented by net/ntoh.c via headers and compiler intrinsics
so declare them in net/Symbol.map.

Reviewed by: imp, kib, emaste
Differential Revision: https://reviews.freebsd.org/D42615

(cherry picked from commit ff3a9d8e2909692c73ce9f9279cac2f15848b3f4)