Fixes to Valgrind for LinuxBrew

Conditioned the flag for `--build=amd64-darwin` on the host being OS X, otherwise the formula fails.  A few  OS X version checks were also conditioned on the OS being OS X, this is mainly for aesthetic reasons.
For a smaller diff, which makes merging easier. (note: untested)
```ruby
    if MacOS.prefer_64_bit?
      args << "--enable-only64bit"
      args << "--build=amd64-darwin" if OS.mac?
    else
      args << "--enable-only32bit"
    end
```