libc: Consolidate unified definitions

As supporting architectures aside from AArch32 and AArch64 is not a
concern, keeping identical definitions in two places for a large part
of the libc seems counterproductive

The int128 types were left un-unified as __int128 is not supported by
gcc on AArch32

Signed-off-by: Bence Szépkúti <bence.szepkuti@arm.com>
Change-Id: Idf08e6fab7e4680d9da62d3c57266ea2d80472cf