Fix tst-mallocfork test

This test was changed from using fork to vfork by:
https://cgit.uclibc-ng.org/cgi/cgit/uclibc-ng.git/commit/?id=822e4896c1072b9f84b17f4f7bcb7c51d1a57723

This change was wrong because man page says the child of vfork() must ONLY do either:
- a call to one of exec(2)
- a call to _exit(2)
- get killed by a signal

BUT the test case does:
- a call to kill
- and also a call to exit(3) and not _exit(2)

This test produces a double free() causing assert in free because this is called several times:
https://elixir.bootlin.com/uclibc-ng/latest/source/libc/stdlib/_atexit.c#L270

So, it seems wrong both in theory (according to man page) and in fact in practice (assert in free).

Here I propose to get back to testing fork(), which is the original test case from:
https://sourceware.org/bugzilla/show_bug.cgi?id=838

And to just "PASS" on no-mmu systems.

For the record, here is the assert call stack:

FAIL tst-mallocfork got 1 expected 0
	./tst-mallocfork: libc/stdlib/malloc-standard/malloc.c: 149: __do_check_inuse_chunk: Assertion `((((mchunkptr)(((char*)(p))+((p)->size & ~0x1)))->size) & 0x1)' failed.
	Didn't expect signal from child: got `Aborted'

0  free (mem=0x3a010) at libc/stdlib/malloc-standard/free.c:285
1  0x00000000000220c4 in __exit_handler (status=1) at libc/stdlib/_atexit.c:270
2  0x000000000001afa0 in __GI_exit (rv=1) at libc/stdlib/_atexit.c:301
3  0x0000000000010dcc in main (argc=1, argv=0x7ffffffd98) at ../test-skeleton.c:405
4  0x000000000001cba8 in __uClibc_main (main=0x10988 <main>, argc=1, argv=0x7ffffffd98, app_init=0x100e8, app_fini=0x23918 <_fini>, rtld_fini=0x0 <k1c_start>, stack_end=0x7ffffffd90) at libc/misc/internals/__uClibc_main.c:512
5  0x00000000000eb8b8 in ?? ()

Signed-off-by: Yann Sionneau <ysionneau@kalray.eu>

test/malloc/tst-mallocfork.c

@@ -9,10 +9,12 @@
 #include <sys/wait.h>
 #include "../test-skeleton.h"
 
+#ifdef __ARCH_USE_MMU__
+
 static void
 sig_handler (int signum)
 {
-  pid_t child = vfork ();
+  pid_t child = fork ();
   if (child == 0)
     exit (0);
   TEMP_FAILURE_RETRY (waitpid (child, NULL, 0));
@@ -35,7 +37,7 @@ do_test (void)
     }
 
   /* Create a child that sends the signal to be caught.  */
-  pid_t child = vfork ();
+  pid_t child = fork ();
   if (child == 0)
     {
       if (kill (parent, SIGALRM) == -1)
@@ -48,5 +50,16 @@ do_test (void)
   return 0;
 }
 
+#else
+
+static int
+do_test (void)
+{
+  printf("Skipping test on non-mmu host!\n");
+  return EXIT_SUCCESS;
+}
+
+#endif
+
 #define TEST_FUNCTION do_test ()
 #include "../test-skeleton.c"