[TSAN] Fix tsan on FreeBSD after D54889

[arichardson]

Summary

It appears that since https://reviews.llvm.org/D54889, BackgroundThread()
crashes immediately because cur_thread()-> will return a null pointer
which is then dereferenced. I'm not sure why I only see this issue on
FreeBSD and not Linux since it should also be unintialized on other platforms.

compiler-rt/trunk/lib/tsan/rtl/tsan_rtl.cpp

static void MemoryProfiler(Context *ctx, fd_t fd, int i) {
   WriteToFile(fd, buf.data(), internal_strlen(buf.data()));
 }
 
 static void BackgroundThread(void *arg) {
   // This is a non-initialized non-user thread, nothing to see here.
   // We don't use ScopedIgnoreInterceptors, because we want ignores to be
   // enabled even when the thread function exits (e.g. during pthread thread
   // shutdown code).
+  cur_thread_init();
   cur_thread()->ignore_interceptors++;
   const u64 kMs2Ns = 1000 * 1000;
 
   fd_t mprof_fd = kInvalidFd;
   if (flags()->profile_memory && flags()->profile_memory[0]) {
     if (internal_strcmp(flags()->profile_memory, "stdout") == 0) {
       mprof_fd = 1;
     } else if (internal_strcmp(flags()->profile_memory, "stderr") == 0) {

compiler-rt/trunk/test/tsan/ignored-interceptors-mmap.cpp

 // RUN: %clangxx_tsan -O0 %s -o %t
 // RUN: not %run %t        2>&1 | FileCheck %s --check-prefix=CHECK-RACE
 // RUN:     %run %t ignore 2>&1 | FileCheck %s --check-prefix=CHECK-IGNORE
-// XFAIL: freebsd,netbsd
+// XFAIL: netbsd
 
 #include <sys/mman.h>
 #include <string.h>
 #include <assert.h>
 #include <atomic>
 
 #include "test.h"