arch-power: Fix stack layout for 64-bit execution

arch-power: Fix stack layout for 64-bit execution

This fixes the call stack layout by changing the size of the
auxiliary vector entries, each of which contain two 64-bit
values. Also, all base addresses for stack contents are now
considered to be 64 bits in order to prevent underflows during
program execution.

Users can now run statically-linked 64-bit ELF ABI v2 compliant
PowerPC LSB ELF executables in syscall emulation mode.

Change-Id: I256399d9344b1b101385e32ad8978325aec9844e
Signed-off-by: Sandipan Das <sandipan@linux.vnet.ibm.com>