clang: Update documentation

As of clang-5.0, things have changed a bit.  First, we cannot
automatically guess -target values as if we do not pass one with CC then
cc-option will fail.  Second, to disable movt/movw relocations the
argument has become -mno-movt.

Related to the target part, we cannot use arm-none-eabi as that ends up
being too generic of an ARM target for things like say rpi_3_32b to
work.

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/arch/arm/config.mk b/arch/arm/config.mk
index 9c213b8..b448ed0 100644
--- a/arch/arm/config.mk
+++ b/arch/arm/config.mk
@@ -23,9 +23,8 @@
       $(call cc-option,-mshort-load-bytes,$(call cc-option,-malignment-traps,))
 
 # LLVM support
-LLVMS_RELFLAGS		:= $(call cc-option,-mllvm,) \
-			$(call cc-option,-target arm-none-eabi,) \
-			$(call cc-option,-arm-use-movt=0,)
+LLVM_RELFLAGS		:= $(call cc-option,-mllvm,) \
+			$(call cc-option,-mno-movt,)
 PLATFORM_RELFLAGS	+= $(LLVM_RELFLAGS)
 
 PLATFORM_CPPFLAGS += -D__ARM__