arm64: Default to building compat vDSO with clang when CONFIG_CC_IS_CLANG

Rather than force the use of GCC for the compat cross-compiler, instead
extract the target from CROSS_COMPILE_COMPAT and pass it to clang if the
main compiler is clang.
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>

arch/arm64/Makefile

@@ -53,7 +53,11 @@ $(warning Detected assembler with broken .inst; disassembly will be unreliable)
   endif
 endif
 
+ifeq ($(CONFIG_CC_IS_CLANG), y)
+COMPATCC ?= $(CC) --target=$(notdir $(CROSS_COMPILE_COMPAT:%-=%))
+else
 COMPATCC ?= $(CROSS_COMPILE_COMPAT)gcc
+endif
 export COMPATCC
 
 ifeq ($(CONFIG_GENERIC_COMPAT_VDSO), y)