[AVR] Generate 'rcall' instead of 'call' on avr2 and avr25

The 'call' (long call) instruction is available on avr3 and above,
and devices in avr2 and avr25 should use the 'rcall' (short call)
instruction for function calls.

Reviewed By: aykevl, dylanmckay

Differential Revision: https://reviews.llvm.org/D121539