x86: Move call64 into its own section

When this code is not used (e.g. by TPL) we want it to be excluded from
the image. Put it in its own section so that this happens.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/x86/cpu/i386/call64.S b/arch/x86/cpu/i386/call64.S
index 275063c..0ffc100 100644
--- a/arch/x86/cpu/i386/call64.S
+++ b/arch/x86/cpu/i386/call64.S
@@ -11,6 +11,7 @@
 #include <asm/processor-flags.h>
 
 .code32
+.section .text_call64
 .globl cpu_call64
 cpu_call64:
 	/*