preempt_forbid_cnt: make it of type cpuatomic_t

This takes care of archs such as the AVR, where int is bigger than the
word size, and updating preempt_forbid_cnt might actually result in two
independent writes.

git-svn-id: https://src.develer.com/svnoss/bertos/trunk@1760 38d2e660-2303-0410-9eaa-f027e97ec537