or1k: Avoid write outside setjmp buf & shrink buf

Update the offsets used to save registers into the stejmp jmp_buf
structure in order to:

* Avoid writing the supervision register outside the buffer and thus
  clobbering something on the stack. Previously the supervision register
  was written at offset 124 while the buffer was of length 124.

* Shrink the jmp_buf down to the size actually needed, by avoiding holes
  at the locations of omitted registers.