testsuite, ubsan: Add libstdc++ deps where required.

We use the ubsan tests from both C, C++, D and Fortran.
thee sanitizer libraries link to libstdc++.

When we are using the C/gdc/gfortran driver, and the target might
require a path to the libstdc++ (e.g. for handing -static-xxxx or
for embedded runpaths), we need to add a suitable option (or we get
fails at execution time because of the missing paths).

Conversely, we do not want to add multiple instances of these
paths (since that leads to failures on tools that report warnings
for duplicate runpaths).

This patch modifies the _init function to allow a sigle parameter
that determines whether the *asan_init should add a path for
libstdc++ (yes for C driver, no for C++ driver).
gcc/testsuite/ChangeLog:

	* g++.dg/ubsan/ubsan.exp:Add a parameter to init to say that
	we expect the C++ driver to provide paths for libstdc++.
	* gcc.dg/ubsan/ubsan.exp: Add a parameter to init to say that
	we need a path added for libstdc++.
	* gdc.dg/ubsan/ubsan.exp: Likewise.
	* gfortran.dg/ubsan/ubsan.exp: Likewise.
	* lib/ubsan-dg.exp: Handle a single parameter to init that
	requests addition of a path to libstdc++ to link flags.