Hi,

This patch adds the (incomplete) support for function and variable
attributes. The added attributes are the ones we're using in
rustc_codegen_gcc but all the groundwork is done to add more (and we
will very likely add more as we didn't add all the ones we use in
rustc_codegen_gcc yet).

The only big question with this patch is about `inline`. We currently
handle it as an attribute because it is more convenient for us but is
it ok or should we create a separate function to mark a function as
inlined?

Thanks in advance for the review.