arch-power: Add multi-mode support

arch-power: Add multi-mode support

This adds multi-mode support and allows the simulator to
read, interpret and execute 32bit and 64-bit, big and
little endian binaries in syscall emulation mode.

During process initialization, a minimal set of hardware
capabilities are also advertised by the simulator to show
support for 64-bit mode and little endian byte order.
This also adds some fixups specific to 64-bit ELF ABI v1
that readjust the entry point and symbol table due to the
use of function descriptors.

Change-Id: I124339eff7b70dbd14e50ff970340c88c13bd0ad
Signed-off-by: Sandipan Das <sandipan@linux.ibm.com>