arch/x86/lib: Fix CONFIG_X86_64 usage in zimage.c

Correct the preprocessor directive used to check for 64-bit kernel
support in the `zboot_go` function. The code previously checked for
`CONFIG_X86_RUN_64BIT`, which is not the correct configuration option
for determining if the kernel should run in 64-bit mode. The correct
option is `CONFIG_X86_64`.

Signed-off-by: Jeremy Compostella <jeremy.compostella@intel.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/x86/lib/zimage.c b/arch/x86/lib/zimage.c
index 65bccda..8f1c18e 100644
--- a/arch/x86/lib/zimage.c
+++ b/arch/x86/lib/zimage.c
@@ -423,7 +423,7 @@
 
 	entry = bmi->load_address;
 	image_64bit = false;
-	if (IS_ENABLED(CONFIG_X86_RUN_64BIT) &&
+	if (IS_ENABLED(CONFIG_X86_64) &&
 	    (hdr->xloadflags & XLF_KERNEL_64)) {
 		image_64bit = true;
 	}