Previously HAVE_IB_PAUSE_INSTRUCTION was never defined and thus InnoDB
never used the PAUSE instruction on non-windows even if it was available.
Probably the check was never migrated from autotools'
storage/innobase/plug.in to storage/innobase/CMakeLists.txt.

Since the check for PAUSE is done at top-level configure.cmake we can
use the result from there (HAVE_PAUSE_INSTRUCTION) instead of rolling
InnoDB's own HAVE_IB_PAUSE_INSTRUCTION (the check is identical anyway).