build: Remove CPU feature config options

build: Remove CPU feature config options

This change removes the Kconfig options selecting CPU feature set (i.e.,
`-march=` compiler flag) from the configuration of this library.
CPU feature flags are set elsewhere in the build configuration and the
intrinsics library can already seamlessly pick up these features via
preprocessor defines.

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