arch/ppc64: Add arch as supported by the clang compiler

Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Change-Id: I1aacff869663e1db74cd485787d7103b9ec5602e
Reviewed-on: https://review.coreboot.org/c/coreboot/+/78448
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Martin L Roth <gaumless@gmail.com>

src/arch/ppc64/Kconfig

@@ -2,6 +2,7 @@
 
 config ARCH_PPC64
 	bool
+	select ARCH_SUPPORTS_CLANG
 
 config ARCH_BOOTBLOCK_PPC64
 	bool