[Sanitizer] Fix GetRSS on Linux with non-4k pages

Summary:
The numbers in /proc/self/statm are in pages, not in fixed 4k units.
This fixes Linux/hard_rss_limit_mb_test.cc on my PowerPC64 box which
has 64k pages.

Reviewers: kcc, willschm

Reviewed By: willschm

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D6717