Kconfig: Reverse ARCH_SUPPORTS_CLANG

Since most targets support clang it's easier to reverse the semantics of
the Kconfig options.

Change-Id: Ib28e7a4cb286b9f8b05be94dae3947179f43c746
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/81659
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Nico Huber <nico.h@gmx.de>
Reviewed-by: Maximilian Brune <maximilian.brune@9elements.com>

src/Kconfig

@@ -74,7 +74,7 @@ config COMPILER_GCC
 
 config COMPILER_LLVM_CLANG
 	bool "LLVM/clang"
-	depends on ALLOW_EXPERIMENTAL_CLANG || ARCH_SUPPORTS_CLANG
+	depends on ALLOW_EXPERIMENTAL_CLANG || !CLANG_UNSUPPORTED
 	help
 	  Use LLVM/clang to build coreboot.  To use this, you must build the
 	  coreboot version of the clang compiler.  Run the command
@@ -85,15 +85,15 @@ config COMPILER_LLVM_CLANG
 
 endchoice
 
-config ARCH_SUPPORTS_CLANG
+config CLANG_UNSUPPORTED
 	bool
 	help
-	  Opt-in flag for architectures that generally work well with CLANG.
-	  By default the option would be hidden.
+	  Set this flag on platforms that do not support building with the
+	  clang compiler.
 
 config ALLOW_EXPERIMENTAL_CLANG
 	bool "Allow experimental LLVM/Clang"
-	depends on !ARCH_SUPPORTS_CLANG
+	depends on CLANG_UNSUPPORTED
 	help
 	  On some architectures CLANG does not work that well.
 	  Use this only to try to get CLANG working.

src/arch/arm/Kconfig

@@ -2,6 +2,7 @@
 
 config ARCH_ARM
 	bool
+	select CLANG_UNSUPPORTED
 
 config ARCH_BOOTBLOCK_ARM
 	bool

src/arch/arm64/Kconfig

@@ -2,6 +2,7 @@
 
 config ARCH_ARM64
 	bool
+	select CLANG_UNSUPPORTED
 
 config ARCH_BOOTBLOCK_ARM64
 	bool

src/arch/ppc64/Kconfig

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

src/arch/riscv/Kconfig

@@ -10,7 +10,6 @@ config ARCH_RISCV_RV32
 
 config ARCH_RISCV
 	bool
-	select ARCH_SUPPORTS_CLANG
 
 if ARCH_RISCV
 

src/arch/x86/Kconfig

@@ -5,7 +5,6 @@ config ARCH_X86
 	select PCI
 	select RELOCATABLE_MODULES
 	select HAVE_ASAN_IN_RAMSTAGE
-	select ARCH_SUPPORTS_CLANG
 
 if ARCH_X86