arch/riscv: Add clang as supported architecture

All emulated targets properly compile and boot to the same extent as
with gcc.

Change-Id: I11ddd9347c2638fb7c26cd4939aa96ff8ddd1e66
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/74571
Reviewed-by: Felix Singer <service+coreboot-gerrit@felixsinger.de>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Elyes Haouas <ehaouas@noos.fr>
Reviewed-by: Daniel Maslowski <info@orangecms.org>

src/arch/riscv/Kconfig

@@ -8,6 +8,7 @@ config ARCH_RISCV_RV32
 
 config ARCH_RISCV
 	bool
+	select ARCH_SUPPORTS_CLANG
 
 if ARCH_RISCV