Add logging syscall

It would be nice to be able to emit logs from the application in the same style the kernel uses.

I have implemented a logging syscall for this purpose:
```
pub unsafe extern "C" fn sys_log(
level: usize,
target_ptr: *const u8,
target_len: usize,
message_ptr: *const u8,
message_len: usize,
)
```
Log filtering is handled by the application to enable compile time log filtering.

If desired, I'll open a PR to upstream this.