Add compatibility with GCC <= 11.2

Add compatibility with GCC <= 11.2

GCC versions prior to 11.3 do not support the "mwait" target attribute.
This change adds compiler version checks to the pragmas using this
attribute, enabling builds with GCC 11.1 & 11.2.

Signed-off-by: Andrei Tatar <andrei@unikraft.io>
Reviewed-by: Stefan Jumarea <stefanjumarea02@gmail.com>
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: #4