gdb/record: add support to AVX unpack instructions

gdb/record: add support to AVX unpack instructions

This commit adds support to recording instructions to unpack high
or low data from XMM registers, identified by the mnemonics in the
form: VPUNPCK [L|H] [BW|WD|DQ|QDQ].
All these instructions are encoded the exact same way, and only affect
the destination register, making them trivial to implement together.

It also updates the test gdb.reverse/i386-avx-reverse.exp to test these
new instructions.  The test always uses ymm because the vpunpck
instructions overwrite the high bits, so we have to be able to record
the full ymm register, not just the output size.

Approved-By: Tom Tromey <tom@tromey.com>