util: Further simplify the initParam implementation.

util: Further simplify the initParam implementation.

Take advantage of string comparisons when looking up what to do with a
given key.

Convert the key_str[12] registers from little endian to host endian.
This matches a corresponding change in the m5 utility to pack the
registers in little endian order, regardless of what the actual guest
endianness is.

Absorb the initparam_keys.hh header into sim/pseudo_inst.cc, and convert
its constants to c++ strings. The constants defined in it might be
useful to guest code calling into the m5 ops, but not for gem5 itself.
By merging them into the .cc file, we also don't have to do any tricks to
try to avoid them having multiple definitions.

Change-Id: I3a450ad7f9c4dca25f79c7835d7f9c167c02ae98
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/27230
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>