Take __weak out of the compiler namespace

With Xcode 4 on OS X shipping LLVM by default, I tried building fio with
clang and found that the definition of __weak conflicts with something
clang defines.  I've attached a patch which replaces the double
underscores with a single underscore, removing it from the
compiler-dependant namespace.

Signed-off-by: Jens Axboe <jaxboe@fusionio.com>