Provide `syscall()` symbol

Provide `syscall()` symbol

Provides the `syscall()` function symbol (some language runtimes  link
to the symbol directly). The function is calling `uk_syscall()` when
lib/syscall_shim is enabled. Otherwise, `syscall()` returns always
`-1` and sets `errno` to `ENOSYS`.

Signed-off-by: Simon Kuenzer <simon.kuenzer@neclab.eu>
Reviewed-by: Felipe Huici <felipe.huici@neclab.eu>