Save some space

- do not put a jump to our initialization into the tiny table.
  Instead, reorder it a bit so that the applications reset vector jump is
  first. This saves 14 bytes.

- in rjmp2add, do not "and" with 0xFFF.
  addr2rjmp will do that anyways, so this is redundant. Saves 4 bytes.

Size now: 2156 bytes.
Backward compat stuff

- gcc < 4.5 does not know about "__builtin_unreachable()" and does not
  need it.

- avr libc < 1.7 (?) does not provide vector number macros.