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

[MaskRay]

Summary

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 are better ways preventing accidental system header inclusion
we can consider it again.

compiler-rt/lib/tsan/CMakeLists.txt

add_sanitizer_rt_symbols(clang_rt.tsan_cxx
       EXTRA rtl/tsan.syms.extra)
     add_dependencies(tsan clang_rt.tsan-${arch}
                           clang_rt.tsan_cxx-${arch}
                           clang_rt.tsan-${arch}-symbols
                           clang_rt.tsan_cxx-${arch}-symbols)
   endforeach()
 endif()
 
-# Make sure that non-platform-specific files don't include any system headers.
-# FreeBSD/NetBSD do not install a number of Clang-provided headers for the
-# compiler in the base system due to incompatibilities between FreeBSD/NetBSD's
-# and Clang's versions. As a workaround do not use --sysroot=. on FreeBSD/NetBSD
-# until this is addressed.
-if(COMPILER_RT_HAS_SYSROOT_FLAG AND NOT CMAKE_SYSTEM_NAME MATCHES "FreeBSD"
-   AND NOT CMAKE_SYSTEM_NAME MATCHES "NetBSD")
-  file(GLOB _tsan_generic_sources rtl/tsan*)
-  file(GLOB _tsan_platform_sources rtl/tsan*posix* rtl/tsan*mac*
-                                   rtl/tsan*linux*)
-  list(REMOVE_ITEM _tsan_generic_sources ${_tsan_platform_sources})
-  set_source_files_properties(${_tsan_generic_sources}
-    PROPERTIES COMPILE_FLAGS "--sysroot=.")
-endif()
-
 # Build libcxx instrumented with TSan.
 if(COMPILER_RT_LIBCXX_PATH AND
    COMPILER_RT_LIBCXXABI_PATH AND
    COMPILER_RT_TEST_COMPILER_ID STREQUAL "Clang" AND
    NOT ANDROID)
   set(libcxx_tsan_deps)
   foreach(arch ${TSAN_SUPPORTED_ARCH})
     get_target_flags_for_arch(${arch} TARGET_CFLAGS)