binutils/testsuite: handle the different install names of c++filt

c++filt is always named cxxfilt in a build directory, but in a install
directory it would be named either cxxfilt or c++filt (depending on
the host).  Handle this last case in testsuite.

binutils/ChangeLog:
        *  testsuite/config/default.exp (CXXFILE): if cxxfilt not found,
        try c++filt.