Bypass potential libc's sysconf interceptors

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

2nd attempt, now with glibc version checks (D31092 was reverted).

Reviewers: eugenis

Subscribers: kubamracek, llvm-commits

Differential Revision: https://reviews.llvm.org/D31221

llvm-svn: 298613