Explicitly specify target for Darwin Intel build

By default uses the target of the system where the script is executed.
If run darwin build on MacOS M1 laptop, it generates 2 arm binaries.
And command `lito` returns error that both files have same arch.

Reproduction of the problem:

```
$ llvm-config --host-target
arm64-apple-darwin23.0.0

$ make crystal stats=true release=true
Using /usr/local/bin/llvm-config [version= 15.0.7]
CRYSTAL_CONFIG_BUILD_COMMIT="1f592eca6" CRYSTAL_CONFIG_PATH='$ORIGIN/../share/crystal/src' SOURCE_DATE_EPOCH="1699639716"  CRYSTAL_CONFIG_LIBRARY_PATH= ./bin/crystal build --no-debug -D strict_multi_assign ....

$ file .../crystal/embedded/bin/crystal
.../crystal/embedded/bin/crystal: Mach-O 64-bit executable arm64
```

Author: miry

omnibus/config/software/crystal.rb

@@ -58,8 +58,9 @@
   copy "#{Dir.pwd}/crystal-#{ohai['os']}-#{ohai['kernel']['machine']}/embedded/bin/crystal", ".build/crystal"
 
   # Compile for Intel
-  command "make crystal stats=true release=true FLAGS=\"#{crflags}\" CRYSTAL_CONFIG_LIBRARY_PATH= O=#{output_path}", env: env
+  command "make crystal stats=true release=true target=x86_64-apple-darwin FLAGS=\"#{crflags}\" CRYSTAL_CONFIG_LIBRARY_PATH= O=#{output_path}", env: env
   move output_bin, "#{output_bin}_x86_64"
+  block { raise "Could not build crystal x86_64" unless File.exist?("#{output_bin}_x86_64") }
 
   # Clean up
   make "clean_cache clean", env: env
@@ -77,14 +78,15 @@
 
   command "clang #{output_path}/crystal.o -o #{output_bin}_arm64 -target arm64-apple-darwin src/llvm/ext/llvm_ext.o `llvm-config --libs --system-libs --ldflags 2>/dev/null` -lstdc++ -lpcre2-8 -lgc -lpthread -levent -liconv -ldl -v", env: env
   delete "#{output_path}/crystal.o"
+  block { raise "Could not build crystal arm64" unless File.exist?("#{output_bin}_arm64") }
 
   # Lipo them up
   command "lipo -create -output #{output_bin} #{output_bin}_x86_64 #{output_bin}_arm64"
   delete "#{output_bin}_x86_64"
   delete "#{output_bin}_arm64"
 
   block do
-    raise "Could not build crystal" unless File.exists?(output_bin)
+    raise "Could not build crystal" unless File.exist?(output_bin)
 
     if macos? || mac_os_x?
       otool_libs = `otool -L #{output_bin}`