[sanitizer] fix asan on the platforms where the page size is not a compile-time constant (or at least where EXEC_PAGESIZE may not be trustworthy), e.g. powerpc64. 

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@182351 91177308-0d34-0410-b5e6-96231b3b80d8