efi/libstub: arm: support building with clang

When building with CONFIG_EFI and CONFIG_EFI_STUB on ARM, the libstub
Makefile would use -mno-single-pic-base without checking it was
supported by the compiler. As the ARM (32-bit) clang backend does not
support this flag, the build would fail.

This changes the Makefile to check the compiler's support for
-mno-single-pic-base before using it, similar to c1c38668 ("ARM:
8767/1: add support for building ARM kernel with clang").
Signed-off-by: Alistair Strachan <astrachan@google.com>
Reviewed-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>

drivers/firmware/efi/libstub/Makefile

@@ -16,7 +16,8 @@ cflags-$(CONFIG_X86)		+= -m$(BITS) -D__KERNEL__ -O2 \
 cflags-$(CONFIG_ARM64)		:= $(subst -pg,,$(KBUILD_CFLAGS)) -fpie \
 				   $(DISABLE_STACKLEAK_PLUGIN)
 cflags-$(CONFIG_ARM)		:= $(subst -pg,,$(KBUILD_CFLAGS)) \
-				   -fno-builtin -fpic -mno-single-pic-base
+				   -fno-builtin -fpic \
+				   $(call cc-option,-mno-single-pic-base)
 
 cflags-$(CONFIG_EFI_ARMSTUB)	+= -I$(srctree)/scripts/dtc/libfdt