ruby: add "x86_64-darwin" and "arm64-darwin" platform gems

Using RCD for these platforms unifies the Darwin native gem build
process with the Linux native gems, which should help avoid
inconsistencies in packaging that result in issues like the missing
Ruby 3.0 binaries in grpc#25060.

Please note that this change leaves the "universal-darwin" platform
native gem untouched, but provides a path forward if the project ever
decides to drop "universal" binary support.

Related to:

- grpc#25429
- grpc#25755
- grpc#25756

Author: flavorjones

Rakefile

@@ -30,6 +30,7 @@ Rake::ExtensionTask.new('grpc_c', spec) do |ext|
   ext.cross_platform = [
     'x86-mingw32', 'x64-mingw32',
     'x86_64-linux', 'x86-linux',
+    'x86_64-darwin', 'arm64-darwin',
     'universal-darwin'
   ]
   ext.cross_compiling do |spec|
@@ -143,7 +144,7 @@ task 'gem:native' do
     # Truncate grpc_c.*.ruby files because they're for Windows only.
     File.truncate('grpc_c.32.ruby', 0)
     File.truncate('grpc_c.64.ruby', 0)
-    ['x86_64-linux', 'x86-linux'].each do |plat|
+    ['x86_64-linux', 'x86-linux', 'x86_64-darwin', 'arm64-darwin'].each do |plat|
       run_rake_compiler(plat, <<~EOT)
         gem update --system --no-document && \
         bundle && \

src/ruby/ext/grpc/extconf.rb

@@ -19,6 +19,7 @@
 bsd = RUBY_PLATFORM =~ /bsd/
 darwin = RUBY_PLATFORM =~ /darwin/
 linux = RUBY_PLATFORM =~ /linux/
+cross_compiling = ENV['RCD_HOST_RUBY_VERSION'] # set by rake-compiler-dock in build containers
 
 grpc_root = File.expand_path(File.join(File.dirname(__FILE__), '../../../..'))
 
@@ -39,7 +40,7 @@
     ENV['LD'] = ENV['CC']
 end
 
-if darwin
+if darwin && !cross_compiling
   ENV['AR'] = 'libtool'
   ENV['ARFLAGS'] = '-o'
 end
@@ -49,7 +50,7 @@
 ENV['EMBED_CARES'] = 'true'
 
 ENV['ARCH_FLAGS'] = RbConfig::CONFIG['ARCH_FLAG']
-if darwin
+if darwin && !cross_compiling
   if RUBY_PLATFORM =~ /arm64/
     ENV['ARCH_FLAGS'] = '-arch arm64'
   else
@@ -102,7 +103,7 @@
 create_makefile(output)
 
 strip_tool = RbConfig::CONFIG['STRIP']
-strip_tool = 'strip -x' if darwin
+strip_tool += ' -x' if darwin
 
 if grpc_config == 'opt'
   File.open('Makefile.new', 'w') do |o|

third_party/rake-compiler-dock/rake_arm64-darwin/Dockerfile

@@ -0,0 +1 @@
+FROM larskanis/rake-compiler-dock-mri-arm64-darwin:1.1.0

third_party/rake-compiler-dock/rake_x86_64-darwin/Dockerfile

@@ -0,0 +1 @@
+FROM larskanis/rake-compiler-dock-mri-x86_64-darwin:1.1.0