[tsan] Fix "failed to intercept sysctlnametomib" on FreeBSD

The sysctlnametomib function is called from __tsan::Initialize via
__sanitizer::internal_sysctlbyname (see stack trace below). This results
in a fatal error since sysctlnametomib has not been intercepted yet.
This patch allows internal_sysctlbyname to be called before
__tsan::Initialize() has completed. On FreeBSD >= 1300045 sysctlbyname()
is a real syscall, but for older versions it calls sysctlnametomib()
followed by sysctl(). To avoid calling the intercepted version, look up
the real sysctlnametomib() followed by internal_sysctl() if the
syscall is not available.

This reduces check-sanitizer failures from 62 to 11 for me.

==34433==FATAL: ThreadSanitizer: failed to intercept sysctlnametomib
    at /exports/users/alr48/sources/upstream-llvm-project/compiler-rt/lib/sanitizer_common/sanitizer_termination.cpp:51
    name=0x7fffffffce10, namelenp=0x7fffffffce08)
    at /exports/users/alr48/sources/upstream-llvm-project/compiler-rt/lib/tsan/../sanitizer_common/sanitizer_common_interceptors.inc:7908
    oldp=0x7fffffffcf2c, oldlenp=0x7fffffffcf20, newp=0x0, newlen=0)
    at /exports/users/alr48/sources/upstream-llvm-project/compiler-rt/lib/sanitizer_common/sanitizer_linux.cpp:803
    at /exports/users/alr48/sources/upstream-llvm-project/compiler-rt/lib/sanitizer_common/sanitizer_linux.cpp:2152
    at /exports/users/alr48/sources/upstream-llvm-project/compiler-rt/lib/tsan/rtl/tsan_rtl.cpp:367
    fname=0x21c731 "readlink", pc=34366042556)
    at /exports/users/alr48/sources/upstream-llvm-project/compiler-rt/lib/tsan/rtl/tsan_interceptors_posix.cpp:255
    bufsiz=1024)
    at /exports/users/alr48/sources/upstream-llvm-project/compiler-rt/lib/tsan/../sanitizer_common/sanitizer_common_interceptors.inc:7151

Reviewed By: #sanitizers, vitalybuka

Differential Revision: https://reviews.llvm.org/D85292