lib/syscall_shim: Add `uk_syscall_do_` syms for execenv registrations

lib/syscall_shim: Add `uk_syscall_do_` syms for execenv registrations

Because of the unnecessary conditional during syscall symbols
generation, execenv syscall registrations would not benefit from
having their `uk_syscall_do_` symbol generated automatically.
Fix this by removing this conditional.

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