libc: Pick up changes from the TF-A code base

This patch picks up the following changes from TF-A:
- libc: Unify intmax_t and uintmax_t on AArch32/64
- libc: Consolidate unified definitions
- libc: Consolidate the size_t and NULL definitions
- libc: Fix SIZE_MAX on AArch32

Signed-off-by: Bence Szépkúti <bence.szepkuti@arm.com>
Change-Id: I6f09ffa4731b9016ad972aece28d9a6ea6f91c32
18 files changed