Set x86_64 as supported architecture for clang

This boots on both qemu and real hardware now.

Change-Id: Ibd320059cff575847bbf1844b5bb100312f77916
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/69235
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>

src/arch/x86/Kconfig

@@ -64,6 +64,7 @@ config ARCH_ALL_STAGES_X86_64
 	select ARCH_VERSTAGE_X86_64 if !VBOOT_STARTS_BEFORE_BOOTBLOCK
 	select ARCH_ROMSTAGE_X86_64
 	select ARCH_RAMSTAGE_X86_64
+	select ARCH_SUPPORTS_CLANG
 
 config HAVE_EXP_X86_64_SUPPORT
 	bool