Added the start of opcode emulation for ARM instructions. This class is designed

to be fed 4 callbacks: read/write memory, and read/write registers. After this,
you can tell the object to read an instruction. This will cause the class to read
the PC, and read and instruction. Then you can emulate the instruction by calling
EvaluateInstruction. This will cause the class to figure out exactly what an opcode
does, and call the read/write mem/regs functions with actual values which allows one
to emulate an instruction without running a process, or it allows one to watch the
context information (the memory write is a pushing register 3 onto the stack at offset
12) so it can be used for generating call frame information. This way, in the future,
we will have one class that can be used to emulate instructions and generate our
unwind info from assembly.

llvm-svn: 123998