fix(darwin): provide compiler-rt libraries for darwin builds

[flavorjones]

The clang macro __builtin_available uses ___isOSVersionAtLeast, which is defined in the compiler-rt runtime library. The compiler-rt library is not currently provided in the rake-compiler-dock environment, which means this symbol may end up being unresolved.

grpc/grpc#28271 reports that the symbol ___isOSVersionAtLeast is undefined at runtime. This issue is reproduced with the test at #59:

Fetching gem metadata from https://rubygems.org/.
Resolving dependencies...
Fetching rake 13.0.6
Installing rake 13.0.6
Using bundler 2.2.32
Fetching minitest 5.14.4
Fetching rake-compiler 1.1.1
Fetching rake-compiler-dock 1.1.0
Installing rake-compiler 1.1.1
Installing rake-compiler-dock 1.1.0
Installing minitest 5.14.4
Using rcd_test 1.0.0 from source at `.`
Bundle complete! 5 Gemfile dependencies, 6 gems now installed.
Use `bundle info [gemname]` to see where a bundled gem is installed.
Run options: --seed 46435
dyld: lazy symbol binding failed: Symbol not found: ___isOSVersionAtLeast

  Referenced from: /Users/runner/hostedtoolcache/Ruby/3.0.3/x64/lib/ruby/gems/3.0.0/gems/rcd_test-1.0.0-x86_64-darwin/lib/rcd_test/3.0/rcd_test_ext.bundle
# Running:
  Expected in: flat namespace


.
dyld: Symbol not found: ___isOSVersionAtLeast
  Referenced from: /Users/runner/hostedtoolcache/Ruby/3.0.3/x64/lib/ruby/gems/3.0.0/gems/rcd_test-1.0.0-x86_64-darwin/lib/rcd_test/3.0/rcd_test_ext.bundle
  Expected in: flat namespace

rake aborted!
SignalException: SIGABRT

Tasks: TOP => test
(See full trace by running task with --trace)
Error: Process completed with exit code 1.

This PR includes that test, along with a proposed fix which is to add compiler-rt to the darwin build environments.

[flavorjones]

Additional context on this problem and the fix is in this series of blog posts:

1. Undefined Symbol on macOS – Eugene Petrenko
2. Undefined isOSVersionAtLeast on macOS – Eugene Petrenko
3. Understanding isOSVersionAtLeast on macOS – Eugene Petrenko

And some prior instances of this exact problem (cross-compiling via osxcross):

• A few issues building Qt · Issue #187 · tpoechtrager/osxcross
• ___isOSVersionAtLeast undefined on OSX · Issue #1325 · ddnet/ddnet

[flavorjones]

@larskanis I think this change might warrant a 1.1.1 release, WDYT?