Continue without DEX support if libdexfile_external.so fails to load.

libunwindstack may be used in situations where we cannot guarantee that
libdexfile_external.so is available, e.g. from libc_malloc_debug.so in the
bootstrap Bionic, or in APEXes with incomplete linker configs.

Test: atest libunwindstack_unit_test
Test: atest --host libunwindstack_unit_test
Test: rm -f out/host/linux-x86/lib*/libdexfile_external.so && \
  out/host/linux-x86/nativetest64/libunwindstack_unit_test/libunwindstack_unit_test
  (check that DexFile(s)Test tests fail without abort)
Bug; 139408016
Change-Id: I7eeee77ab363d9d39412ece2038ce786394bb34f