Set PROT_EXEC on the whole pixelflinger code cache.

The pointer difference between word pointers is a number
of words, and it needs to be multiplied by the size of a word
to get a proper byte size.

Without this, we tend to see crashes when the code crosses
a page boundary.

Bug: 3026204
Bug: 3097482
Change-Id: I37776d26d5afcdb1da71680de02fbb95e6548371