linux-user/flatload: fix initial stack pointer alignment

Stack pointer alignment code incorrectly adds stack_size to sp instead
of subtracting it. It also does not take flat_argvp_envp_on_stack() into
account when calculating stack_size. This results in initial stack
pointer misalignment with certain set of command line arguments and
environment variables and correct alignment for the same binary with a
different set of arguments. This misalignment causes failures in the
following tests in the testsuite of gcc built for xtensa uclinux:

  gcc.dg/torture/vshuf-v64qi.c
  gcc.dg/torture/vshuf-v8sf.c
  gcc.dg/torture/vshuf-v8si.c
Reviewed-by: NLaurent Vivier <lvivier@redhat.com>
Signed-off-by: NMax Filippov <jcmvbkbc@gmail.com>

linux-user/flatload.c

@@ -771,10 +771,10 @@ int load_flt_binary(struct linux_binprm *bprm, struct image_info *info)
     /* Enforce final stack alignment of 16 bytes.  This is sufficient
        for all current targets, and excess alignment is harmless.  */
     stack_len = bprm->envc + bprm->argc + 2;
-    stack_len += 3;	/* argc, arvg, argp */
+    stack_len += flat_argvp_envp_on_stack() ? 2 : 0; /* arvg, argp */
+    stack_len += 1; /* argc */
     stack_len *= sizeof(abi_ulong);
-    if ((sp + stack_len) & 15)
-        sp -= 16 - ((sp + stack_len) & 15);
+    sp -= (sp - stack_len) & 15;
     sp = loader_build_argptr(bprm->envc, bprm->argc, sp, p,
                              flat_argvp_envp_on_stack());