target-xtensa: fix big-endian BBS/BBC implementation

Quote from ISA, 2.1:

For most Xtensa instructions, bit numbering is irrelevant; only the BBC
and BBS instructions assign bit numbers to values on which the processor
operates. The BBC/BBS instructions use big-endian bit ordering (0 is the
most-significant bit) on a big-endian processor configuration.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>