arch: Stop using TheISA within the ISAs.

We know for sure what the ISA is, so there's no need for the
indirection.

Change-Id: I73ff04c50890d40a4c7f40caeee746b68b846cb3
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/18488
Reviewed-by: Brandon Potter <Brandon.Potter@amd.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>

arch: Use const StaticInstPtr references where possible

This patch optimises the passing of StaticInstPtr by avoiding copying
the reference-counting pointer. This avoids first incrementing and
then decrementing the reference-counting pointer.

power: Remove unused private members to fix compile-time warning

Certain versions of clang complain about unused private members if
they are not used. This changeset removes such members from the
POWER-specific ProcessInfo struct to silence the warning.

trace: reimplement the DTRACE function so it doesn't use a vector
At the same time, rename the trace flags to debug flags since they
have broader usage than simply tracing. This means that
--trace-flags is now --debug-flags and --trace-help is now --debug-help

Make commenting on close namespace brackets consistent.

Ran all the source files through 'perl -pi' with this script:

s|\s*(};?\s*)?/\*\s*(end\s*)?namespace\s*(\S+)\s*\*/(\s*})?|} // namespace $3|;
s|\s*};?\s*//\s*(end\s*)?namespace\s*(\S+)\s*|} // namespace $2\n|;
s|\s*};?\s*//\s*(\S+)\s*namespace\s*|} // namespace $1\n|;

Also did a little manual editing on some of the arch/*/isa_traits.hh files
and src/SConscript.

POWER: Add support for the Power ISA

This adds support for the 32-bit, big endian Power ISA. This supports both
integer and floating point instructions based on the Power ISA Book I v2.06.