M68000: Implement BKPT opcode

[vkoskiv]

This enables BKPT (Breakpoint) instructions to show up in the decompiler in the same way as trap is shown - with a new pcode __m68k_bkpt(#)

Technically, this instruction was only added in the 68010. From the Motorola manual:

For the MC68000 and MC68008, the breakpoint cycle is not run, but an illegal
instruction exception is taken.

For the MC68010, a breakpoint acknowledge bus cycle is run with function codes driven high and zeros on all address lines. Whether the breakpoint acknowledge bus cycle is terminated with DTACK, BERR, or VPA, the processor always takes an illegal instruction exception. During exception processing, a debug monitor can distinguish different software breakpoints by decoding the field in the BKPT instruction.

It may therefore be more correct to specify this only for 68010 and newer, and only show ILLEGAL on 68000, but Ghidra currently makes no distinction between the 000 and 010, so I opted to keep this PR minimal by not adding the multiple files needed. This patch already enables a better decompile of affected code.

[ryanmkurtz]

An equivalent fix has been incorporated into 197a18a