Both cmd_pkt and data_buf class members are cast to a uint16_t pointer,
so we need to make sure they are properly aligned, otherwise it is
undefined behavior.
Result of running IWYU (https://include-what-you-use.org/) and
applying most of the suggestions about unncessary includes and
forward declarations.
Was motivated by observing that <thread> was being included in
ppcopcodes.cpp even though it was unused (found while researching
the use of threads), but seems generally good to help with build
times and correctness.
