arm64: vdso32: Pass '--target' option to clang via VDSO_CAFLAGS

Directly passing the '--target' option to clang by appending to
COMPATCC does not work if COMPATCC has been specified explicitly as
an argument to Make unless the 'override' directive is used, which is
ugly and different to what is done in the top-level Makefile.

Move the '--target' option for clang out of COMPATCC and into
VDSO_CAFLAGS, where it will be picked up when compiling and assembling
the 32-bit vDSO under clang.
Reported-by: Catalin Marinas <catalin.marinas@arm.com>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>

arch/arm64/kernel/vdso32/Makefile

@@ -10,7 +10,7 @@ include $(srctree)/lib/vdso/Makefile
 
 # Same as cc-*option, but using COMPATCC instead of CC
 ifeq ($(CONFIG_CC_IS_CLANG), y)
-COMPATCC ?= $(CC) --target=$(notdir $(CROSS_COMPILE_COMPAT:%-=%))
+COMPATCC ?= $(CC)
 else
 COMPATCC ?= $(CROSS_COMPILE_COMPAT)gcc
 endif
@@ -38,6 +38,10 @@ VDSO_CPPFLAGS += $(LINUXINCLUDE)
 # Common C and assembly flags
 # From top-level Makefile
 VDSO_CAFLAGS := $(VDSO_CPPFLAGS)
+ifneq ($(shell $(COMPATCC) --version 2>&1 | head -n 1 | grep clang),)
+VDSO_CAFLAGS += --target=$(notdir $(CROSS_COMPILE_COMPAT:%-=%))
+endif
+
 VDSO_CAFLAGS += $(call cc32-option,-fno-PIE)
 ifdef CONFIG_DEBUG_INFO
 VDSO_CAFLAGS += -g