[PROF-15559] Add headers for Ruby 2.6 to 3.2 + bump gem version to 3.5.5

[ivoanjo]

What does this PR do?

This PR adds (trimmed) headers for Ruby 2.6 to 3.2.

Motivation:

We're dropping our use of the MJIT headers, and just use the "vendored headers" approach for all Rubies in dd-trace-rb. This will allow us to clean up a number of awkward separate code paths, and in the future should open the door to supporting single step instrumentation (SSI) for the profiler as well.

Additional Notes:

I had opened an earlier version of this PR as a draft. It seems all's fine, so let's move forward.

How to test the change?

This PR pairs with DataDog/dd-trace-rb#6135 for dd-trace-rb to use these headers. Since dd-trace-rb has an hardcoded list of Ruby versions that use MJIT we can release the change on this gem first, and then land the dd-trace-rb side after.

[eregon]

Looks good.
Will need to be rebased on master and rerun add_source + find_includes.rb since #30 requires more sources but also fixed the script which removes more files.

[ivoanjo]

Hey @eregon can you give a new pass? I've

• Rebased on top of master + re-did the imports with the latest version of the script
• Added the missing 3.2 headers
• Bumped the version so we're ready to release this update

[eregon]

Nice.

We need to make sure this doesn't break existing datadog gem releases though (they need to still not use these headers on versions where the MJIT header is included), have you checked that?

[ivoanjo]

We need to make sure this doesn't break existing datadog gem releases though (they need to still not use these headers on versions where the MJIT header is included), have you checked that?

Yup:

• I packaged the gem, and used it with version 2.41.0 of the gem without issues on Ruby 3.0.7 -- it still used the MJIT header.

• This is fine because the decision to use the MJIT header is static, it's not based on any kind of probing of header availability, see:

  • https://github.com/DataDog/dd-trace-rb/blob/2d0fbaf95f95f344ffe123f849dded4e910683ae/ext/datadog_profiling_native_extension/extconf.rb#L249
  • https://github.com/DataDog/dd-trace-rb/blob/2d0fbaf95f95f344ffe123f849dded4e910683ae/ext/datadog_profiling_native_extension/native_extension_helpers.rb#L13

[ivoanjo]

I've force-pushed a rebase on top of master with no changes to re-sign the commits, as the ones I had done in my workspace were missing the signature.

[eregon]

Realized now:

datadog-ruby_core_source/.github/workflows/test-compilation.yml

Lines 28 to 32 in d1b0b24

	- { ruby_version: '2.6', expected_exit_code: 0 } # using MJIT headers
	- { ruby_version: '2.7', expected_exit_code: 0 } # using MJIT headers
	- { ruby_version: '3.0', expected_exit_code: 0 } # using MJIT headers
	- { ruby_version: '3.1', expected_exit_code: 0 } # using MJIT headers
	- { ruby_version: '3.2', expected_exit_code: 0 } # using MJIT headers

needs an update, the comment is outdated (also git grep -i MJIT) but also we should test both 2.6.0 and 2.6 like it's done for 3.4, and same for other versions changed.