arch-power: Restore consistency with other platforms

arch-power: Restore consistency with other platforms

The 32-bit POWER reference test binary was removed in c1ebdf66f
(as a nasty surprise for POWER users).

The remaining platforms split between two approaches:

MIPS rebuilds "hello" from source.
This fails for two reasons:
1) The trivial reason is that on POWER make abends due to no makefile.
2) The more fundamental reason is that gem5 is not completely bug-free
(especially the Decoder on POWER in this case), therefore regression
testing is only possible if we have not just some hello program, but
a very particular bit sequence to serve as an immutable reference.

ARM and X86 follow the reference-bit-sequence approach.  POWER will
be consistent with same.  Including the sha1 for hello32,
77b27b67393311546e768b5ff35202490bad71aa, as a simple immutability
assurance.  I have also renamed hello to hello32 in anticipation to
merge Sandipan's e52dbcb.

Change-Id: I77ef31349c9e50b987c6f58bb23324844527366d
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/40635
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Sandipan Das <sandipan@linux.ibm.com>
Reviewed-by: Pratik Sampat <pratik.r.sampat@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>