sim/testsuite/cris: Remove faulty use of basename in C tests

Calls to basename were added here as part of commit
e1e1ae6e "sim: testsuite: fix objdir handling", but that
commit missed adding "#include <libgen.h>" or the equivalent
GNU extension, see basename(3).  Fixing that shows a logical
error in the change to openpf1.c; the non-/-prefixed
code-path was changed instead of the "/"-prefixed code-path,
which is the one executed after that commit.

For "newlib" these tests failed linking after that commit.
Recent newlib has the (asm-renamed) GNU-extension-variant of
basename, but we're better off not using it at all.

Unfortunately, compilation failures for C tests run by the
machinery in c.exp are currently just marked "unresolved",
in contrast to C and assembler tests run by calling
run_sim_test.

The interaction of calling with the full program-path vs.
use of --sysroot exposes a consistency problem: when
--sysroot is used, argv[0] isn't the path by which the
program can find itself.  It's undecided whether argv[0] for
the program running in the simulator should be edited
(related to the naked argument to the simulator before
passing on to the simulated program) to remove a leading
--sysroot.  Either way, such a change would be out of scope
for this commit.

	* c/stat3.c (mybasename): New macro.  Use it instead of basename.
	* c/openpf1.c: Correct basename-related change and update related
	comment.