Add assembler, macro-assembler and disassembler support for AArch32

Add support for the A32 and T32 instruction set architectures in
seperate a32 directories.

Note that this commit introduces aarch32 and aarch64 namespaces so
existing client code will need to be adjusted.

Refer to doc/getting-started-a32.md for an introduction.

Change-Id: Iaf3e5f496ec4e19d77d304128e6920daa4549e78