Kconfig: Mark clang as ready to use on some arch

This adds 2 flags:
* invisible opt-in flag for platforms on which clang seems to work
* visible opt-in flag to allow experimenting

Clang seems to work rather well on x86_32 so it makes sense to start
adding that to Jenkins buildtesting, which this allows.

This allows abuild to differentiate between targets that are known to
build with clang. This makes buildtesting just those targets easier.

Change-Id: I46f1bad59bda94f60f4a141237ede11f6eb93cc2
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/63081
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Martin L Roth <gaumless@tutanota.com>

src/arch/x86/Kconfig

@@ -33,6 +33,7 @@ config ARCH_ALL_STAGES_X86_32
 	select ARCH_VERSTAGE_X86_32
 	select ARCH_ROMSTAGE_X86_32
 	select ARCH_RAMSTAGE_X86_32
+	select ARCH_SUPPORTS_CLANG
 
 # stage selectors for x64