Allow RISCV to be compiled with ANY_TOOLCHAIN

Change-Id: I9210241c902ad8a88980a7c9cdb0d52c460b2541
Signed-off-by: Ronald G. Minnich <rminnich@gmail.com>
Reviewed-on: http://review.coreboot.org/8025
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
Tested-by: build bot (Jenkins)

src/Kconfig

@@ -280,6 +280,7 @@ config ARCH_ARM64
 config ARCH_RISCV
 	bool
 	default n
+	select ANY_TOOLCHAIN
 
 source src/arch/x86/Kconfig
 source src/arch/arm/Kconfig