add jemalloc support

[piperswe]

jemalloc is often a more optimal memory allocator for Ruby workloads.

[bobvanderlinden]

I overlooked this PR. Looks like a good change, thanks! It'll take a while for CI to run.

[bobvanderlinden]

Bleh, both ruby-2.3.4 and ruby-2.3.1 are failing the build (tried multiple times 😅). It segfaults during the build:

2023-04-22T22:29:23.3250290Z ruby> generating enc.mk
2023-04-22T22:29:23.3499940Z ruby> /build/ruby-2.3.4/lib/fileutils.rb:1748: [BUG] Segmentation fault at 0x00000000000022
2023-04-22T22:29:23.3502725Z ruby> ruby 2.3.4p301 (2017-03-30 revision 58214) [x86_64-linux]

We can make patches for these specific versions or excempt these versions from being tested.

I'm also fine with not testing this at all. It doubles the build times for new versions and more chances of causing trouble for me maintaining it 😅. If someone wants to use the jemalloc it's their choice, the option is available, but the binaries aren't cached nor tested.

[bobvanderlinden]

Support for ruby-2.3 has been dropped due to other issues. The tests are now passing for all supported versions 👍