Bypass potential libc's sysconf wrappers for sysconf(_SC_PAGESIZE) call

Summary:
sysconf(_SC_PAGESIZE) is called very early, during sanitizer init and
any instrumented code (a wrapper/interceptor will likely be instrumented)
calling back to sanitizer before init is done will most surely crash.

Reviewers: eugenis

Subscribers: llvm-commits, kubamracek

Differential Revision: https://reviews.llvm.org/D31092

llvm-svn: 298305