Some of the macro defines in the header files of `include/lib/stdlib/machine/`
folder are not correct for AArch32. This patch fixes the same.

Change-Id: I8bfaf638a7986fd902648d2074537bd26c313cb3
Signed-off-by: Soby Mathew <soby.mathew@arm.com>

* Move stdlib header files from include/stdlib to include/lib/stdlib for
  consistency with other library headers.
* Fix checkpatch paths to continue excluding stdlib files.
* Create stdlib.mk to define the stdlib source files and include directories.
* Include stdlib.mk from the top level Makefile.
* Update stdlib header path in the fip_create Makefile.
* Update porting-guide.md with the new paths.

Change-Id: Ia92c2dc572e9efb54a783e306b5ceb2ce24d27fa