Fix issue with old rubygems not detecting musl linux properly

[ivoanjo]

What does this PR do?

Fix issue with old rubygems not detecting musl linux properly.

Old versions of rubygems (for instance 3.0.3) don't properly detect alternative libc implementations on Linux; in particular for our case, they don't detect musl.

(For reference, Rubies older than 2.7 may have shipped with an affected version of rubygems).

In such cases, we fall back to use RbConfig::CONFIG['arch'] instead.

Why not use RbConfig::CONFIG['arch'] always? Because Gem::Platform.local.to_s does some normalization we want in other situations -- for instance, it turns x86_64-linux-gnu to
x86_64-linux. So for now we only add this workaround in a specific situation where we actually know it is wrong.

See also rubygems/rubygems#2922 and rubygems/rubygems#4082

Fixes DataDog/dd-trace-rb#2222

Motivation

Fix DataDog/dd-trace-rb#2222; customers can manually upgrade to a non-buggy rubygems, but having this fixed on our side provides a better user experience.

Additional Notes

I'll ask someone working on Ruby to review this change, since it's very Ruby-specific.

How to test the change?

The ruby:2.5.9-alpine3.13 docker image ships with an affected version of Rubygems 3.0.3. Without this fix, the following command will print the wrong (non-musl) path for libdatadog:

$ docker run --network=host -ti -v `pwd`:/working ruby:2.5.9-alpine3.13 /bin/sh
/ # gem -v
3.0.3
/ # gem install libdatadog
Fetching libdatadog-0.7.0.1.0-x86_64-linux.gem
Successfully installed libdatadog-0.7.0.1.0-x86_64-linux
1 gem installed
/ # ruby -e "require 'libdatadog'; pp Libdatadog.pkgconfig_folder"
"/usr/local/bundle/gems/libdatadog-0.7.0.1.0-x86_64-linux/vendor/libdatadog-0.7.0/x86_64-linux/libdatadog-x86_64-unknown-linux-gnu/lib/pkgconfig"

with the new libdatadog (here I install it manually -- you can run bundle exec rake package to replicate locally), the correct version gets used:

/ # gem install working/libdatadog-0.7.0.1.1-x86_64-linux.gem
Successfully installed libdatadog-0.7.0.1.1-x86_64-linux
1 gem installed
/ # gem -v
3.0.3
/ # ruby -e "require 'libdatadog'; pp Libdatadog.pkgconfig_folder"
"/usr/local/bundle/gems/libdatadog-0.7.0.1.1-x86_64-linux/vendor/libdatadog-0.7.0/x86_64-linux-musl/libdatadog-x86_64-alpine-linux-musl/lib/pkgconfig"