libc: centralize ntoh symbols

[brooks]

Summary

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

[emaste]

Yes these shouldn't have been MD

[kib]

lib/libc/net/Symbol.map

69

Shouldn't 'ntoh*' placed in the 'n' block of the version?

[brooks]

lib/libc/net/Symbol.map

69

This file (for good or ill) seem to group things by function and only loosely alphabetically. For example see the get* above with related end* and set*.