Use the same name as glibc & gnulib to indicate "newlib itself is
being compiled". This also harmonizes the codebase a bit in that
_LIBC was already used in places instead of _COMPILING_NEWLIB.
Building for bfin-elf, mips-elf, and x86_64-pc-cygwin produces
the same object code.
Bump GPLv2+ to GPLv3+ for some files, clarify BSD 2-clause.
Everything else stays under GPLv3+.
New Linking Exception exempts resulting executables from LGPLv3 section 4.
Add CONTRIBUTORS file to keep track of licensing.
Remove 'Copyright Red Hat Inc' comments.
Signed-off-by: Corinna Vinschen <corinna@vinschen.de>