Use write instead of read permissions to check for global sections on mac

Summary:
The LINKEDIT section is very large and is read-only. Scanning this
section caused LSan on darwin to be very slow. When only writable sections
are scanned for global pointers, performance improved by a factor of about 25x.

Reviewers: alekseyshl, kubamracek

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D33322

llvm-svn: 303422