[XTENSA] fix bit operations in bitops.h

A macro used internally in the bitops.h file to identify the number of
leading zeros (__cntlz) was implemented incorrectly for Xtensa processors
that don't provide the 'nsa/nsau' instructions. As a result, the dependent
functions and macros (ffs, ffz, fls) were broken.

Signed-off-by: Chris Zankel <chris@zankel.net>