Support -malign-double option

While LLVM supports the `-malign-double` option, Clang does not.
Passing in `-mllvm -malign-double` cannot be used as a workaround
as it will result in different machine description being used
by Clang and LLVM.

This change adds support for `-malign-double` option directly into
Clang and sets the machine description accordingly.

BUG=none
R=dschuff@chromium.org

Review URL: https://codereview.chromium.org/1310173005 .