Bring the powerpc DDB disassembler into the 21st century

Bring in the most recent copy of NetBSD's db_disasm, to fix bugs and add more
instructions.

• Fix several bugs in the disassembler, most notably the disassembly of the rlwi* instructions, the original reason for bringing in this change.
• Add more registers to the SPR list
• Add more instructions to the opcode table

Obtained from: NetBSD
MFC after: 2 weeks