powerpc: Kconfig: disable CONFIG_COMPAT for clang < 12

Until clang-12, clang would attempt to assemble 32b powerpc assembler in
64b emulation mode when using a 64b target triple with -m32, leading to
errors during the build of the compat VDSO. Simply disable all of
CONFIG_COMPAT; users should upgrade to the latest release of clang for
proper support.
Suggested-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://github.com/ClangBuiltLinux/linux/issues/1160
Link: https://github.com/llvm/llvm-project/commits/2288319733cd5f525bf7e24dece08bfcf9d0ff9e
Link: https://groups.google.com/g/clang-built-linux/c/ayNmi3HoNdY/m/XJAGj_G2AgAJ
Link: https://lore.kernel.org/r/20210518205858.2440344-1-ndesaulniers@google.com

arch/powerpc/Kconfig

@@ -289,6 +289,7 @@ config PANIC_TIMEOUT
 config COMPAT
 	bool "Enable support for 32bit binaries"
 	depends on PPC64
+	depends on !CC_IS_CLANG || CLANG_VERSION >= 120000
 	default y if !CPU_LITTLE_ENDIAN
 	select ARCH_WANT_OLD_COMPAT_IPC
 	select COMPAT_OLD_SIGACTION