Add architecture option that switches x86/x64 powershell

Allow to select x86 or x64 powershell to execute commands.

The architecture (`:x86_64` or `:i386`) can be specified by RSpec(SpecInfra) configuration or metadata (`:86_64` is default).

```ruby
# specified by RSpec configuration
RSpec.configure do |c|
  c.architecture = :x86_64
end

# specified by metadata
describe windows_registry_key('HKEY_LOCAL_MACHINE\SOFTWARE\SOME_SOFTWEARE'), architecture: :i386 do
  # actually access 'HKLM\SOFTWARE\Wow6432Node\SOME_SOFTWARE' in x64 OS
  ...
end
```