Modified the BUILD.gn files in order to be able to easily switch
between Subzero and LLVM back ends. Note that Subzero
compilation should already work on Linux, as soon as the Wheezy
to Jessie update lands. For now, only Windows will build Subzero.

Also removed linux hack from libEGL build file.

Change-Id: Ic584b604496c4b4f745b05d72a752f056495055b
Reviewed-on: https://swiftshader-review.googlesource.com/8630Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>