arch/x86/gdt: Move variable assignment down

The Clang Static Analyzer warns about an unused assignment, when
building the image for the Lenovo X60.

```
src/arch/x86/gdt.c:39:6: warning: Value stored to 'num_gdt_bytes' \
        during its initialization is never read
        u16 num_gdt_bytes = (uintptr_t)&gdt_end - (uintptr_t)&gdt;
            ^~~~~~~~~~~~~   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
```

If `CONFIG_RELOCATABLE_RAMSTAGE` is selected, the function returns
before the assignment is used. So, move the assignment below the if
statement.

Change-Id: Ibcb8bce743d8cb3625647804816fb97f937dc429
Found-by: clang version 4.0.1-6 (tags/RELEASE_401/final), Debian Sid/unstable
Signed-off-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-on: https://review.coreboot.org/21957
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Sumeet R Pawnikar <sumeet.r.pawnikar@intel.com>
Reviewed-by: Martin Roth <martinroth@google.com>
diff --git a/src/arch/x86/gdt.c b/src/arch/x86/gdt.c
index b7bc62e..24a1c81 100644
--- a/src/arch/x86/gdt.c
+++ b/src/arch/x86/gdt.c
@@ -37,7 +37,7 @@
 static void move_gdt(int is_recovery)
 {
 	void *newgdt;
-	u16 num_gdt_bytes = (uintptr_t)&gdt_end - (uintptr_t)&gdt;
+	u16 num_gdt_bytes;
 	struct gdtarg gdtarg;
 
 	/* ramstage is already in high memory. No need to use a new gdt. */
@@ -45,6 +45,7 @@
 		return;
 
 	newgdt = cbmem_find(CBMEM_ID_GDT);
+	num_gdt_bytes = (uintptr_t)&gdt_end - (uintptr_t)&gdt;
 	if (!newgdt) {
 		newgdt = cbmem_add(CBMEM_ID_GDT, ALIGN(num_gdt_bytes, 512));
 		if (!newgdt) {