This commit is the result of running the gdb/copyright.py script,
which automated the update of the copyright year range for all
source files managed by the GDB project to be updated to include
year 2023.
Simplify the build by moving the generation of these files from
build-time (via dgen.c that we have to compile & execute on the
build system) to maintainer/release mode (via spreg-gen.py that
we only ever execute when the spreg table actually changes). It
speeds up the build process and makes it easier for us to reason
about & review changes to the code generator.
The tool is renamed from "dgen" because it's hardcoded to only
generated spreg files. It isn't a generalized tool for creating
lookup tables.
