arch-power: Add MSR and associated dependencies

arch-power: Add MSR and associated dependencies

This adds the definition of the Machine State Register
(MSR) in preparation for multi-mode support. The MSR
has bits that define the state of the processor. This
defines all the bitfields and sets the ones that are
typically used for userspace environments.

In preparation for multi-mode support, the SF and LE
bits are used by instructions to check if the simulation
is running in 64-bit mode and if memory accesses are to
be performed in little endian byte order respectively.
This introduces changes in areas such as target address
computation for branch instructions, carry and overflow
computation for arithmetic instructions, etc.

Change-Id: If9ac69415ca85b0c873bd8579e7d1bd2219eac62
Signed-off-by: Sandipan Das <sandipan@linux.ibm.com>