[libc] Rearrange error and signal tables.

This is largely a cosmetic change done with a few goals:
1. Reduce the conditionals in picking the correct set of tables for the
   platform.
2. Avoid exposing, for example Linux errors, when building for non-Linux
   platforms. This also prevents build failures when Linux errors are not
   defined on the target non-Linux platform.
3. Some "_table" suffixes have been removed to avoid repeated
   occurance of "table" like "tables/linux_error_table.h".

Reviewed By: michaelrj

Differential Revision: https://reviews.llvm.org/D151367