Support additional instructions in ARM and thumb assemblers

This adds the following support for the ARM and thumb assemblers:

1. Shifting by a register.
2. LDR/STR with a register offset, possibly shifted.
3. LDR(literal).
4. STR PC relative.

Also adds tests for them in the thumb assembler gtest.

Change-Id: Ie467e3c1d06b699cacbdef3482ed9a92e4f1809b
8 files changed