lib/posix-process: Add `libc` system call wrapper for `clone`

lib/posix-process: Add `libc` system call wrapper for `clone`

In order to successfully call the clone system call through a libc
wrapper we need to be able to have an assembly sequence that both
translates the arguments passed by the libc wrapper to those of the
underlying syscall, since their signatures differ, as well as redirect
the parent and child accordingly upon exit from said syscall: child
must run its requested function and argument and parent must return
to the clone caller with an unscathed register context.

Said assembly sequence must be aware of the signature differences
between architectures, e.g. ARM64 vs x86_64 signatures of the
clone system call.

Signed-off-by: Sergiu Moga <sergiu@unikraft.io>
Approved-by: Michalis Pappas <michalis@unikraft.io>
Reviewed-by: Michalis Pappas <michalis@unikraft.io>
Reviewed-by: Andrei Tatar <andrei@unikraft.io>
GitHub-Closes: #1618