i#3092 genapi: Rename and clean up opcode headers

[derekbruening]

Moves the small non-public parts of core/ir/*/opcode.h into instr.h
and opnd.h.

Cleans up core/ir/*/opcode.h and codec.py's generated content such
that these files are now just directly copied to the public include/
directory.

Renames opcode.h to opcode_api.h for all 3 arches to make it clear
they are part of the public interface.

Issue: #3092

[derekbruening]

Github has once again messed up 32-bit libraries on their VM's:

2021-04-01T16:11:18.1127287Z 8: [100%] Linking C executable bin/linux.execve32
2021-04-01T16:11:18.1128546Z 8: /usr/lib/x86_64-linux-gnu/libdl.so: error adding symbols: File in wrong format
2021-04-01T16:11:18.1129378Z 8: collect2: error: ld returned 1 exit status

2021-04-01T16:07:58.4961762Z [  2%] Linking C executable bin/linux.infinite
2021-04-01T16:07:58.4962874Z /usr/lib/x86_64-linux-gnu/libdl.so: error adding symbols: File in wrong format
2021-04-01T16:07:58.4964490Z suite/tests/CMakeFiles/linux.infinite.dir/build.make:98: recipe for target 'suite/tests/bin/linux.infinite' failed
2021-04-01T16:07:58.4966284Z CMakeFiles/Makefile2:7501: recipe for target 'suite/tests/CMakeFiles/linux.infinite.dir/all' failed
2021-04-01T16:07:58.4967132Z collect2: error: ld returned 1 exit status

2021-04-01T16:07:58.4971965Z [  2%] Linking C executable bin/client.mbr_instrumentation_segment
2021-04-01T16:07:58.4973167Z /usr/lib/x86_64-linux-gnu/libdl.so: error adding symbols: File in wrong format
2021-04-01T16:07:58.4974711Z suite/tests/CMakeFiles/client.mbr_instrumentation_segment.dir/build.make:98: recipe for target 'suite/tests/bin/client.mbr_instrumentation_segment' failed

2021-04-01T16:09:28.5436729Z [ 58%] Linking CXX executable ../bin32/prefetch_analyzer_launcher
2021-04-01T16:09:28.5438665Z /usr/lib/x86_64-linux-gnu/libz.so: error adding symbols: File in wrong format
2021-04-01T16:09:28.5440235Z clients/drcachesim/CMakeFiles/prefetch_analyzer_launcher.dir/build.make:120: recipe for target 'clients/bin32/prefetch_analyzer_launcher' failed
2021-04-01T16:09:28.5441277Z collect2: error: ld returned 1 exit status

And another package change breaking us:

Reading Script: /home/runner/work/dynamorio/dynamorio/suite/runsuite.cmake,automated_ci;64_only;require_format
Detected a CI clang suite: disabling running of tests
CMake Error at /home/runner/work/dynamorio/dynamorio/suite/runsuite.cmake:254 (message):
  FATAL ERROR: clang-format is required but not found!

None of these are related to the PR so going ahead and asking for the review while we try to work around these GA CI breakages.

[derekbruening]

Filed the GA problems as #4830. Will try to resolve/work around to get this PR green.

[derekbruening]

Not proving easy to fix the VM's. Waiting on reply from GA devs at actions/runner-images#2950 (comment)

Going to go ahead and put this in. The 64-bit build and tests all passed except the cross-arch one; confidence enough there.