[CMake][tsan] Remove --sysroot=.

rL254966 added `--sysroot=.` to prevent accidental including system headers.
It caused hassle to FreeBSD (D17383)/NetBSD. The next problem is that
we want to include `features.h` (usually `/usr/include/features.h`) to detect `__GLIBC__`.

At this point it seems that `--sysroot=.` adds lots of inconvenience so we disable it for now.
If there is a better way preventing accidental system header inclusion we can consider it again.

Reviewed By: #sanitizers, vitalybuka

Differential Revision: https://reviews.llvm.org/D93921