Add XBTS and IBTS instructions (80386 only)

[BarebitOpenSource]

Suggested by fuzxxl:

In early steppings of the 80386, opcodes 0f a6 (xbts) and 0f a7 (ibts) existed. These were removed later on.

Ralf Brown's interrupt list claims that they were present in early versions of the Intel manuals for 80386, googling yielded documentation with timings and behaviour.