util/cbfstool: Support AMD64 rmodules

Add support for 64bit rmodule, as required for relocatable
ramstage on x86_64.

Change-Id: I7fbb3b4c0f76ce82c090b5f16f67a728b6bf94a5
Signed-off-by: Patrick Rudolph <siro@das-labor.org>
Reviewed-on: https://review.coreboot.org/c/29874
Reviewed-by: Philipp Deppenwiese <zaolin.daisuki@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

util/cbfstool/elfheaders.c

@@ -1072,6 +1072,9 @@ static void fixup_relocations(struct elf_writer *ew)
 	case EM_386:
 		type = R_386_32;
 		break;
+	case EM_X86_64:
+		type =  R_AMD64_64;
+		break;
 	case EM_ARM:
 		type = R_ARM_ABS32;
 		break;