Clean up the 'trap' instruction printing a bit. Non-Darwin assemblers don't

(yet) recognize the 'trap' mnemonic, so we use .short/.long to emit the
opcode directly. On Darwin, however, we do want the mnemonic for more
readable assembly code and better disassembly.

Adjust the .td file to use the 'trap' mnemonic and handle using the binutils
workaround in the assembly printer. Also tweak the formatting of the opcode
values to make them consistent between the MC printer and the old printer.

llvm-svn: 114679