disable 'vmstate' builtin when using system's malloc(3)

Related discussion:
https://github.com/ksh93/ksh/issues/95#issuecomment-664010969

src/cmd/ksh93/tests/leaks.sh:
- When ksh is compiled to use the system's malloc(3) instead of AST
  vmalloc(3), the vmstate builtin returns either nothing or zero.
  Detect this as a regression test failure and refuse to run tests.
- Tweak iterations. Tests don't need 500 or 1000 runs for vmstate.

src/cmd/ksh93/data/builtins.c:
- Do not compile in vmstate builtin when using system's malloc(3).

src/cmd/ksh93/data/builtins.c

@@ -156,8 +156,10 @@ const struct shtable3 shtab_builtins[] =
 	CMDLIST(uname)
 	CMDLIST(wc)
 	CMDLIST(sync)
+#if !_std_malloc && !_AST_std_malloc
 	CMDLIST(vmstate)
 #endif
+#endif
 #if SHOPT_REGRESS
 	"__regress__",		NV_BLTIN|BLT_ENV,	bltin(__regress__),
 #endif