cpu/x86/smm: Fix u32 type mismatch in print statement

The 64-bit compiler x86_64-linux-gnu-gcc-10 aborts the build with the
format warning below:

        CC         ramstage/cpu/x86/smm/smm_module_loader.o
    src/cpu/x86/smm/smm_module_loader.c:415:42: error: format '%lx' expects argument of type 'long unsigned int', but argument 4 has type 'u32' {aka 'unsigned int'} [-Werror=format=]
      415 |  printk(BIOS_DEBUG, "%s: stack_end = 0x%lx\n",
          |                                        ~~^
          |                                          |
          |                                          long unsigned int
          |                                        %x
      416 |   __func__, stub_params->stack_top - total_stack_size);
          |             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
          |                                    |
          |                                    u32 {aka unsigned int}

The size of `size_t` differs between i386-elf (32-bit) and
x86_64-elf/x86_64-linux-gnu (64-bit).

Unfortunately, coreboot hardcodes

    src/include/inttypes.h:#define PRIx32  "x"

so `PRIx32` cannot be used.

There use `z` as length modifier, as size_t should be always big enough
to hold the value.

Found-by: x86_64-linux-gnu-gcc-10 (Debian 10.2.1-6) 10.2.1 20210110
Fixes: afb7a814 ("cpu/x86/smm: Introduce SMM module loader version 2")
Change-Id: Ib504bc5e5b19f62d4702b7f485522a2ee3d26685
Signed-off-by: Paul Menzel <pmenzel@molgen.mpg.de>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/54343
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Felix Held <felix-coreboot@felixheld.de>
diff --git a/src/cpu/x86/smm/smm_module_loader.c b/src/cpu/x86/smm/smm_module_loader.c
index 9f0ae92..101b7c5 100644
--- a/src/cpu/x86/smm/smm_module_loader.c
+++ b/src/cpu/x86/smm/smm_module_loader.c
@@ -411,7 +411,7 @@
 
 	const size_t total_stack_size =
 		params->num_concurrent_stacks * params->per_cpu_stack_size;
-	printk(BIOS_DEBUG, "%s: stack_end = 0x%lx\n",
+	printk(BIOS_DEBUG, "%s: stack_end = 0x%zx\n",
 		__func__, stub_params->stack_top - total_stack_size);
 	printk(BIOS_DEBUG,
 		"%s: stack_top = 0x%x\n", __func__, stub_params->stack_top);