sanitizer_common: Try looking up symbols with RTLD_DEFAULT if RTLD_NEXT does not work.

If the lookup using RTLD_NEXT failed, the sanitizer runtime library
is later in the library search order than the DSO that we are trying
to intercept, which means that we cannot intercept this function. We
still want the address of the real definition, though, so look it up
using RTLD_DEFAULT.

Differential Revision: https://reviews.llvm.org/D39779

llvm-svn: 317930