Fix detecting of Apple Silicon architecture

[jacob-carlborg-apoex]

On some Macs, Ruby reports the host CPU as "arm". Might be related to which version of the kernel is installed, but uname still reports "arm64".

This is an extract of RbConfig::CONFIG:

{
  "target_os"=>"darwin21",
  "target_vendor"=>"apple",
  "target_cpu"=>"arm64",
  "target"=>"arm64-apple-darwin21",
  "host_os"=>"darwin21.3.0",
  "host_vendor"=>"apple",
  "host_cpu"=>"arm",
  "host"=>"arm-apple-darwin21.3.0",
  "RUBY_VERSION_NAME"=>"ruby-2.7.0",
  "RUBYW_BASE_NAME"=>"rubyw",
  "RUBY_BASE_NAME"=>"ruby",
  "build_os"=>"darwin21.3.0",
  "build_vendor"=>"apple",
  "build_cpu"=>"arm",
  "build"=>"arm-apple-darwin21.3.0",
}