Fix regression on mcore-elf port after recent switch conversion change

Fix regression on mcore-elf port after recent switch conversion change

Filip's recent change to re-enable switch conversion at -Og triggered a
regression on the mcore-elf target.

If we look at tree-switch-conversion.cc we have this:

  if (flag_pic)
    return false;

The mcore-elf port defines a dummy ASM_OUTPUT_ADDR_DIFF_ELT which is designed
to trigger an assembler syntax error and thus fail loudly.  That definition
comes from a time when it appears we had to define that macro in every port,
even if it wasn't being used.

These days we do not need to define that macro unless it's really needed.  And
a definition like the one for mcore-elf will cause problems
(compile/pr69102.c).  That definition has also been the cause of a long
standing failure in the port (gcc.dg/pr47446-2.c).

Naturally this has been through a round of testing where it fixes the two
issues noted above without any regressions.

gcc/
	* config/mcore/mcore.h (ASM_OUT_ADDR_DIFF_ELT): Remove.