[asan] Use MADV_NOCORE for use_madv_dontdump on FreeBSD.

[MaskRay]

On Linux, I can observe the effect of the option:

ulimit -c 100; rm -f core; ASAN_OPTIONS=abort_on_error=1:disable_coredump=0:use_madv_dontdump=1 ./a; readelf -Wl core | grep 2008f
ulimit -c 100; rm -f core; ASAN_OPTIONS=abort_on_error=1:disable_coredump=0:use_madv_dontdump=0 ./a; readelf -Wl core | grep 2008f

# the file size field is 0 with use_madv_dontdump=1
  LOAD           0x92e000 0x000002008fff7000 0x0000000000000000 0xdfff0001000 0xdfff0001000 RW  0x1000

[khng300]

Just tested it on FreeBSD 11.2, the relevant changes look fine to me.

aa.core-readelf2 KBDownload

compiler-rt/trunk/lib/sanitizer_common/sanitizer_posix_libcdep.cc

 #ifdef MADV_NOHUGEPAGE  // May not be defined on old systems.
   return madvise((void *)addr, size, MADV_NOHUGEPAGE) == 0;
 #else
   return true;
 #endif  // MADV_NOHUGEPAGE
 }
 
 bool DontDumpShadowMemory(uptr addr, uptr length) {
-#ifdef MADV_DONTDUMP
+#if defined(MADV_DONTDUMP)
   return madvise((void *)addr, length, MADV_DONTDUMP) == 0;
+#elif defined(MADV_NOCORE)
+  return madvise((void *)addr, length, MADV_NOCORE) == 0;
 #else
   return true;
 #endif  // MADV_DONTDUMP
 }
 
 static rlim_t getlim(int res) {
   rlimit rlim;
   CHECK_EQ(0, getrlimit(res, &rlim));