Fixed a few things in the BUILD.gn files:
1) Style guide mentions single entries should be on single lines
2) Changed "host_os == ..." for the is_... equivalent values
3) Added missing Mac libs and include dirs

Change-Id: I28d35fd86a8cf9c9157a397370a385fe3e5e958b
Reviewed-on: https://swiftshader-review.googlesource.com/7152Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>