Remove GCC compatibility adaptations

Remove GCC compatibility adaptations

This change removes code changes meant for compatibility with GCC,
reverting the headers to ones released with LLVM 7.0.1.
This is in anticipation of an update of upstream code, as well as a
rework of compiler compatibility.

Signed-off-by: Andrei Tatar <andrei@unikraft.io>
Reviewed-by: Maria Sfiraiala <maria.sfiraiala@gmail.com>
Reviewed-by: Radu Nichita <radunichita99@gmail.com>
Approved-by: Razvan Deaconescu <razvand@unikraft.io>
Tested-by: Unikraft CI <monkey@unikraft.io>
GitHub-Closes: #3