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>

arch/arm/config.mk

@@ -23,9 +23,8 @@ PLATFORM_RELFLAGS += $(call cc-option, -msoft-float) \
       $(call cc-option,-mshort-load-bytes,$(call cc-option,-malignment-traps,))
 
 # LLVM support
-LLVMS_RELFLAGS		:= $(call cc-option,-mllvm,) \
+LLVM_RELFLAGS		:= $(call cc-option,-mllvm,) \
-			$(call cc-option,-target arm-none-eabi,) \
+			$(call cc-option,-mno-movt,)
-			$(call cc-option,-arm-use-movt=0,)
 PLATFORM_RELFLAGS	+= $(LLVM_RELFLAGS)
 
 PLATFORM_CPPFLAGS += -D__ARM__

doc/README.clang

@@ -10,16 +10,11 @@ used to get its value. This does lead to larger code then strictly
 necessary, but at least works.
 
 NOTE: target compilation only work for _some_ ARM boards at the moment.
-Also Aarch64 is not supported: Most notably boards which aren't using
+Also AArch64 is not supported currently due to a lack of private libgcc
-the generic board will fail to compile, but since those are expected
+support.  Boards which reassign gd in c will also fail to compile, but there is
-to be converted this will solve itself. Boards which reassign gd in c
+in no strict reason to do so in the ARM world, since crt0.S takes care of this.
-will also fail to compile, but there is in no strict reason to do so
+These assignments can be avoided by changing the init calls but this is not in
-in the ARM world, since crt0.S takes care of this. These assignments
+mainline yet.
-can be avoided by changing the init calls but this is not in mainline yet.
-
-NOTE: without the -mllvm -arm-use-movt=0 flags U-Boot will compile
-fine, but llvm might hardcode addresses in movw / movt pairs, which
-cannot be relocated and U-Boot will fail at runtime.
 
 Debian (based)
 --------------
@@ -29,7 +24,8 @@ sudo apt-get install clang
 Note that we still use binutils for some tools so we must continue to set
 CROSS_COMPILE. To compile U-Boot with clang on linux without IAS use e.g.:
 make HOSTCC=clang rpi_2_defconfig
-make HOSTCC=clang CROSS_COMPILE=arm-linux-gnueabi- CC=clang -j8
+make HOSTCC=clang CROSS_COMPILE=arm-linux-gnueabi- \
+    CC="clang -target arm-linux-gnueabi" -j8
 
 It can also be used to compile sandbox:
 make HOSTCC=clang sandbox_defconfig