FROMLIST: BACKPORT: Makefile: lld: set -O2 linker flag when linking w…

…ith LLD

For arm64:
0.34% size improvement with lld -O2 over lld for vmlinux.
3.3% size improvement with lld -O2 over lld for Image.lz4-dtb.

Link: ClangBuiltLinux/linux#343
Suggested-by: Rui Ueyama <ruiu@google.com>
Suggested-by: Nathan Chancellor <natechancellor@gmail.com>
Reviewed-by: Nathan Chancellor <natechancellor@gmail.com>
Tested-by: Nathan Chancellor <natechancellor@gmail.com>
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>

Makefile

@@ -647,6 +647,7 @@ endif
 ifeq ($(cc-name),clang)
 ifeq ($(ld-name),lld)
 KBUILD_CFLAGS	+= -fuse-ld=lld
+KBUILD_LDFLAGS	+= -O2
 endif
 KBUILD_CPPFLAGS	+= -Qunused-arguments
 endif