[libc] Add implementation of errno and define the other macros of errno.h.

Reviewers: stanshebs, alexbrachet

Subscribers: mgorny, MaskRay, tschuett, libc-commits

Tags: #libc-project

Differential Revision: https://reviews.llvm.org/D71094

GitOrigin-RevId: 453c85ff0f96048ea31037fed905ef6a06ac3fcc
14 files changed