[libc++] Move __errc to __system_error/errc.h

This file was added before we started granularizing the headers, but is essentially just a granularized header. This moves the header to the correct place.

Reviewed By: #libc, EricWF

Spies: libcxx-commits, arichardson, mikhail.ramalho

Differential Revision: https://reviews.llvm.org/D146395