[libc][NFC] Move the Linux file implementation to a subdirectory

This patch simply moves the special handling for `linux` files to a
subdirectory. This is done to make it easier in the future to extend
this support to targets (like the GPU) that will have different
dependencies.

Reviewed By: lntue

Differential Revision: https://reviews.llvm.org/D151231

GitOrigin-RevId: 1ef0bafc4f6f03805558b900d703292fb957c100
4 files changed