Don't trigger sanitizer initialization from `sysctlbyname` and `sysctl` interceptor.

Summary:
This fixes the `ThreadSanitizer-x86_64-iossim` testsuite which broke
when r348770 (https://reviews.llvm.org/D55473) landed.

The root cause of the problem is that early-on during the iOS simulator
init process a call to `sysctlbyname` is issued. If the TSan initializer
is triggered at this point it will eventually trigger a call to
`__cxa_at_exit(...)`. This call then aborts because the library
implementing this function is not yet had its initialization function
called.

rdar://problem/46696934

Reviewers: kubamracek, george.karpenkov, devnexen, vitalybuka, krytarowski

Subscribers: #sanitizers, llvm-commits

Differential Revision: https://reviews.llvm.org/D55711

llvm-svn: 349402