FROMLIST: BACKPORT: Makefile: lld: tell clang to use lld

This is needed because clang doesn't select which linker to use based on
$LD but rather -fuse-ld={bfd,gold,lld,<absolute path to linker>}.  This
is problematic especially for cc-ldoption, which checks for linker flag
support via invoking the compiler, rather than the linker.

Select the linker via absolute path from $PATH via `which`. This allows
you to build with:

$ make LD=ld.lld
$ make LD=ld.lld-8
$ make LD=/path/to/ld.lld

Add -Qunused-arguments to KBUILD_CPPFLAGS when using LLD, as otherwise
Clang likes to complain about -fuse-lld= being unused when compiling but
not linking (-c) such as when cc-option is used.

Link: ClangBuiltLinux/linux#342
Link: ClangBuiltLinux/linux#366
Link: ClangBuiltLinux/linux#357
Suggested-by: Nathan Chancellor <natechancellor@gmail.com>
Suggested-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>

Makefile

@@ -647,6 +647,13 @@ LLVM_NM		:= llvm-nm
 export LLVM_AR LLVM_NM
 endif
 
+ifeq ($(cc-name),clang)
+ifeq ($(ld-name),lld)
+KBUILD_CFLAGS	+= -fuse-ld=lld
+endif
+KBUILD_CPPFLAGS	+= -Qunused-arguments
+endif
+
 # The arch Makefile can set ARCH_{CPP,A,C}FLAGS to override the default
 # values of the respective KBUILD_* variables
 ARCH_CPPFLAGS :=
@@ -757,7 +764,6 @@ ifdef CONFIG_KCOV
 endif
 
 ifeq ($(cc-name),clang)
-KBUILD_CPPFLAGS += $(call cc-option,-Qunused-arguments,)
 KBUILD_CFLAGS += $(call cc-disable-warning, format-invalid-specifier)
 KBUILD_CFLAGS += $(call cc-disable-warning, gnu)
 KBUILD_CFLAGS += $(call cc-disable-warning, attribute-alias)