Move the libc-i386, libc-amd64, and libc-arm include directories into
the standard "include/spec" directory. This allows the platform specific
headers in the libc API package to be detected in a generic manner.
Ref #3051
This patch changes the top-level directory layout as a preparatory
step for improving the tools for managing 3rd-party source codes.
The rationale is described in the issue referenced below.
Issue #1082