lib/syscall_shim: Add non-LL execenv syscall registration variant

lib/syscall_shim: Add non-LL execenv syscall registration variant

Based on UK_LLSYSCALL_R_E_DEFINE and provides a libc-style wrapper
in case UK_LIBC_SYSCALLS is enabled.

(Similar to its UK_SYSCALL_R_DEFINE, non-execenv variants)

Checkpatch-Ignore: MACRO_ARG_REUSE
Checkpatch-Ignore: COMPLEX_MACRO
Checkpatch-Ignore: SPACING
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