[libc] Make the errno macro resolve to the thread local variable directly.

With modern architectures having a thread pointer and language supporting
thread locals, there is no reason to use a function intermediary to access
the thread local errno value.

The entrypoint corresponding to errno has been replaced with an object
library as there is no formal entrypoint for errno anymore.

Reviewed By: jeffbailey, michaelrj

Differential Revision: https://reviews.llvm.org/D120920