libc: Consolidate the size_t and NULL definitions

Consolidate the definition of size_t to one header per AArch, and
the definition of NULL to one header

Signed-off-by: Bence Szépkúti <bence.szepkuti@arm.com>
Change-Id: Iecfbad2cf360cfb705ce7aaa981700fd16219b82
12 files changed