[libc] Allow target architecture independent configs

Previously, we required entrypoints.txt for every target architecture
supported by a target OS. With this change, we allow architecture
independent config for a target OS. That is, if an architecture specific
entrypoints.txt is missing, then a generic entrypoints.txt for that
target OS will be used.

Reviewed By: caitlyncano

Differential Revision: https://reviews.llvm.org/D105147