Use a simpler workaround for platform mixup issue

When `linux` platform comes first, then a rake task is defined for `lib/2.7/pg_ext.so` which links to `linux` instead of `linux-musl`.
Defining `linux-musl` first fixes this since the task is defined only once in:
  https://github.com/rake-compiler/rake-compiler/blob/ad13dd1ae1000e1ef91b95f3c7782ccca4c1c591/lib/rake/extensiontask.rb#L349

The mixed platform rake definition otherwise leads to a rake dependency chain like this:

```
rake aborted!
Tasks: TOP => pkg/pg-1.6.0-x86_64-linux-musl.gem => lib/2.7/pg_ext.so => copy:pg_ext:x86_64-linux:2.7.8 => tmp/x86_64-linux/pg_ext/2.7.8/pg_ext.so => tmp/x86_64-linux/pg_ext/2.7.8/Makefile
```

Author: larskanis

Rakefile

@@ -49,10 +49,10 @@ CrossLibraries = [
 	['x64-mingw-ucrt', 'mingw64', 'x86_64-w64-mingw32'],
 	['x86-mingw32', 'mingw', 'i686-w64-mingw32'],
 	['x64-mingw32', 'mingw64', 'x86_64-w64-mingw32'],
-	['x86_64-linux', 'linux-x86_64', 'x86_64-linux-gnu'],
 	['x86_64-linux-musl', 'linux-x86_64', 'x86_64-unknown-linux-musl'],
-	['aarch64-linux', 'linux-aarch64', 'aarch64-linux-gnu'],
+	['x86_64-linux', 'linux-x86_64', 'x86_64-linux-gnu'],
 	['aarch64-linux-musl', 'linux-aarch64', 'aarch64-linux-musl'],
+	['aarch64-linux', 'linux-aarch64', 'aarch64-linux-gnu'],
 	['x86_64-darwin', 'darwin64-x86_64', 'x86_64-apple-darwin'],
 	['arm64-darwin', 'darwin64-arm64', 'arm64-apple-darwin'],
 ].map do |platform, openssl_config, toolchain|
@@ -70,13 +70,7 @@ Rake::ExtensionTask.new do |ext|
 	ext.lib_dir        = 'lib'
 	ext.source_pattern = "*.{c,h}"
 	ext.cross_compile  = true
-
-	# Activate current cross compiled platform only.
-	# This is to work around the issue that `linux` platform is selected in `linux-musl` image.
-	ext.cross_platform = CrossLibraries.map(&:platform).select do |pl|
-		m = ENV["RCD_IMAGE"]&.match(/:(?<ruby_ver>[\d\.]+)-mri-(?<platform>[-\w]+)$/)
-		m && m[:platform] == pl
-	end
+	ext.cross_platform = CrossLibraries.map(&:platform)
 
 	ext.cross_config_options += CrossLibraries.map do |xlib|
 		{