gh-142342: Fix m68k assembler operand constraints for `%fpcr` access (gh-142343)

On m68k, an fmove instruction accessing %fpcr may only move from
or to a data register or a memory operand. The constraint "g" also
permits the use of address registers, which is invalid. The correct
constraint is "dm". Beginning with GCC 15, the register allocator
picks an address register in the code which causes SIGILL during
runtime.

Co-authored-by: Michael Karcher <github@mkarcher.dialup.fu-berlin.de>