[asan] Fix deadlock issue on FreeBSD, caused by use of .preinit_array in rL325240

Summary:
Without this patch,
clang -fsanitize=address -xc =(printf 'int main(){}') -o a; ./a => deadlock in __asan_init>AsanInitInternal>AsanTSDInit>...>__getcontextx_size>_rtld_bind>rlock_acquire(rtld_bind_lock, &lockstate)

libexec/rtld-elf/rtld.c
  wlock_acquire(rtld_bind_lock, &lockstate);
  if (obj_main->crt_no_init)
    preinit_main(); // unresolved PLT functions cannot be called here

lib/libthr/thread/thr_rtld.c
  uc_len = __getcontextx_size(); // unresolved PLT function in libthr.so.3

check-xray tests currently rely on .preinit_array so we special case in
xray_init.cc

Subscribers: srhines, kubamracek, krytarowski, delcypher, llvm-commits, #sanitizers

Differential Revision: https://reviews.llvm.org/D48806

llvm-svn: 336067