Load and store context according to trapframe structure
Restore kernel stack upon exception if we were in userland...

and save it when going to user land.
Add support for syscalls

syscall exception handler is written and calls syscall()
which then calls the approriate syscall with correct arguments.

The ABI for LM32 will be:

- syscall number in R1
- arg1 in R2
- arg2 in R3
- and so on...
- syscall return value in R1 (and R2 if > 32 bits)