Allow valgrind to find debug info in a 'usr merge' setup.

Allow valgrind to find debug info in a 'usr merge' setup.

On ubuntu 19.10, valgrind fails telling that it cannot find
the mandatory redirection for strlen in ld-linux-x86-64.so.2.

This is due to /bin being a symlink to usr/bin: ld is found
in /usr/lib/x86_64-linux-gnu/ld-2.30.so
but its debug info is
in /usr/lib/debug/lib/x86_64-linux-gnu/ld-2.30.so

Without this patch, valgrind searches the debug info (a.o.)
in /usr/lib/debug/usr/lib/x86_64-linux-gnu/ld-2.30.so
so using the concatenation of  /usr/lib/debug
and /usr/lib/x86_64-linux-gnu/ld-2.30.so,
but the debug info is located at the concatenation of
/usr/lib/debug and /lib/x86_64-linux-gnu/ld-2.30.so
(so without the leading /usr).
Modify the debug info search so as to try with and without the /usr.

Patch derived from the patch done by Mathieu Trudel-Lapierre
to solve https://bugs.launchpad.net/ubuntu/+source/valgrind/+bug/1808508