powerpc: Add display of raw instruction values to x/I in ddb.

The "alternate format" character 'I' previously had the same behavior as
the "display as an instruction" character 'i'. With this change, it will now
prefix each disassembled instruction with the raw hex value.

As PowerPC instructions are always 32 bits and always aligned, and there are
no alternate modes that would affect instruction decoding or display, this
seemed to me to be the obvious interpretation of "alternate format".

Approved by: jhibbits (mentor)
Differential Revision: https://reviews.freebsd.org/D22223