AVR: Disable tree-switch-conversion per default.

AVR: Disable tree-switch-conversion per default.

There are at least two cases where tree-switch-conversion leads
to unpleasant resource allocation:

PR49857
    The lookup table lives in RAM.  This is the case for all
    devices that locate .rodata in RAM, which is for almost
    all AVR devices.

PR81540
    Code is bloated for 64-bit inputs.

As far as PR49857 is concerned, a target hook that may add an
address-space qualifier to the lookup table is the obvious
solution, though a respective patch has always been rejected by
global maintainers for non-technical reasons.

gcc/
	PR target/81540
	PR target/49857
	* common/config/avr/avr-common.cc: Disable -ftree-switch-conversion.