FROMLIST: Makefile: lld: set -O2 linker flag when linking with 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.

(am from https://lkml.org/lkml/2019/2/11/1949)

Change-Id: Ibdc7ecc9861562305b49456d6d37274ce8075e22
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>
Bug: 63740206
Signed-off-by: Yousef Algadri <yusufgadrie@gmail.com>
Signed-off-by: Raphiel Rollerscaperers <rapherion@raphielgang.org>
Signed-off-by: Chenyang Zhong <zhongcy95@gmail.com>

Makefile

@@ -915,6 +915,10 @@ KBUILD_CFLAGS += $(call cc-disable-warning, unused-function)
 KBUILD_CFLAGS += $(call cc-disable-warning, implicit-function-declaration)
 endif
 
+ifeq ($(ld-name),lld)
+KBUILD_LDFLAGS += -O2
+endif
+
 KBUILD_CFLAGS += $(call cc-disable-warning, unused-const-variable)
 ifdef CONFIG_FRAME_POINTER
 KBUILD_CFLAGS	+= -fno-omit-frame-pointer -fno-optimize-sibling-calls