Allow to override target architecture sufix in archive name

tgz_package uses build platform for naming archives.
When build packages for multiple architectures like x86_64 and arm, then the archive name is not correct.
Update the script of tgz_package to check for configuration option `ohai['target_arch']` as target arch name.

It allows to specify architecture in project like:

```
ohai['target_arch'] = 'universal'
dependency 'tgz_package'
```

Author: miry

.circleci/config.yml

@@ -27,7 +27,7 @@ jobs:
           command: |
             cd omnibus
             bundle check || bundle install --binstubs
-            bundle exec omnibus build llvm
+            bundle exec omnibus build llvm --log-level debug
       - store_artifacts:
           path: ./omnibus/pkg
           destination: llvm

darwin/README.md

@@ -9,12 +9,18 @@ The whole process is automated using a `Makefile`.
 * `pkgconfig`, `libtool` (Can be installed by `$ brew install pkgconfig libtool`)
 * Own `/opt/crystal`, `/var/cache`.
 
-```
+```shell
 sudo mkdir -p /opt/crystal
 sudo chown $(whoami) /opt/crystal/
 sudo mkdir -p /var/cache
 sudo chown $(whoami) /var/cache
 ```
+* Optional: If you need to build LLVM, ensure the existence of the /opt/llvm directory.
+
+```shell
+sudo mkdir -p /opt/llvm
+sudo chown $(whoami) /opt/llvm/
+```
 
 # Getting started
 

omnibus/config/projects/llvm.rb

@@ -18,6 +18,9 @@ def valid_cmake_version?
 
 dependency 'cmake' unless valid_cmake_version?
 dependency 'llvm'
+
+# Requires for tgz_package to generate a proper archive name with sufix universal.
+ohai['target_arch'] = 'universal'
 dependency 'tgz_package' if macos? || mac_os_x? || centos?
 
 exclude '\.git*'

omnibus/config/software/tgz_package.rb

@@ -4,10 +4,12 @@
 
 build do
   block do
+    platform = ohai['os']
+    target_arch = ohai['target_arch'] || ohai['kernel']['machine']
     destination = File.expand_path('pkg', Omnibus::Config.project_root)
     version = "#{project.build_version}-#{project.build_iteration}"
     version.gsub!("/", "-")
-    tgz_name = "#{project.name}-#{version}-#{ohai['os']}-#{ohai['kernel']['machine']}.tar.gz"
+    tgz_name = "#{project.name}-#{version}-#{platform}-#{target_arch}.tar.gz"
     if macos? || mac_os_x?
       transform = "-s /./#{project.name}-#{version}/"
     else