[ASan] Fix large allocator test case to cope with 64k pages

Summary: My PowerPC64 Linux box has 64k pages. The test assumes 4k pages. Fix it.

Reviewers: glider, eugenis, samsonov

Reviewed By: samsonov

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D6228

llvm-svn: 221875