arm64: vdso32: Enable Clang Compilation

Enable Clang Compilation for the vdso32 library.
Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Nathan Chancellor <natechancellor@gmail.com> # build
Tested-by: Stephen Boyd <swboyd@chromium.org>
Reviewed-by: Nathan Chancellor <natechancellor@gmail.com>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Link: https://lkml.kernel.org/r/20200320145351.32292-27-vincenzo.frascino@arm.com

arch/arm64/kernel/vdso32/Makefile

@@ -10,7 +10,18 @@ include $(srctree)/lib/vdso/Makefile
 
 # Same as cc-*option, but using CC_COMPAT instead of CC
 ifeq ($(CONFIG_CC_IS_CLANG), y)
+COMPAT_GCC_TOOLCHAIN_DIR := $(dir $(shell which $(CROSS_COMPILE_COMPAT)elfedit))
+COMPAT_GCC_TOOLCHAIN := $(realpath $(COMPAT_GCC_TOOLCHAIN_DIR)/..)
+
+CC_COMPAT_CLANG_FLAGS := --target=$(notdir $(CROSS_COMPILE_COMPAT:%-=%))
+CC_COMPAT_CLANG_FLAGS += --prefix=$(COMPAT_GCC_TOOLCHAIN_DIR)
+CC_COMPAT_CLANG_FLAGS += -no-integrated-as -Qunused-arguments
+ifneq ($(COMPAT_GCC_TOOLCHAIN),)
+CC_COMPAT_CLANG_FLAGS += --gcc-toolchain=$(COMPAT_GCC_TOOLCHAIN)
+endif
+
 CC_COMPAT ?= $(CC)
+CC_COMPAT += $(CC_COMPAT_CLANG_FLAGS)
 else
 CC_COMPAT ?= $(CROSS_COMPILE_COMPAT)gcc
 endif