Option to stop the diff at the first return / MWCC fix

[ethteck]

Relevant here: https://decomp.me/scratch/wi3NU where some stuff is inlined, but the compiler isn't omitting the inlined functions' code, so the diff gets nasty. We should provide an option to stop at the first return mnemonic or turn on --disassemble for the gc_wii platform

[encounter]

MWCC has no problem inserting several blrs regardless of version, I was wrong. But this hack would still be useful for cases like the above.

I found that asm-differ requires a tweak to make --stop-jr-ra work for PPC (by checking for blr as well), but that can easily be submitted upstream.