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

Calls to basename were added here as part of commit
e1e1ae6e9b5e "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.

sim/testsuite/cris/c/openpf1.c

@@ -3,8 +3,8 @@
 
    We assume, with EXE being the name of the executable:
    - The simulator executes with cwd the same directory where the executable
-     is located (so argv[0] contains a plain filename without directory
-     components).
+     is located (also argv[0] contains a plain filename without directory
+     components -or- argv[0] contains the full non-sysroot path to EXE).
    - There's no /EXE on the host file system.  */
 
 #include <stdio.h>
@@ -21,8 +21,10 @@ int main (int argc, char *argv[])
       if (fnam == NULL)
 	abort ();
       strcpy (fnam, "/");
-      strcat (fnam, basename (argv[0]));
+      strcat (fnam, argv[0]);
     }
+  else
+    fnam = strrchr (argv[0], '/');
 
   f = fopen (fnam, "rb");
   if (f == NULL)

sim/testsuite/cris/c/stat3.c

@@ -7,13 +7,14 @@
 #include <stdio.h>
 #include <string.h>
 #include <stdlib.h>
+#define mybasename(x) ({ const char *x_ = (x), *y_ = strrchr (x_, '/'); y_ != NULL ? y_ + 1 : x_; })
 
 int main (int argc, char *argv[])
 {
   char path[1024] = "/";
   struct stat buf;
 
-  strcat (path, basename (argv[0]));
+  strcat (path, mybasename(argv[0]));
   if (stat (".", &buf) != 0
       || !S_ISDIR (buf.st_mode))
     abort ();