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)

Bug: 63740206
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>
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Signed-off-by: Alexander Winkowski <dereference23@outlook.com>

Makefile

@@ -889,6 +889,10 @@ KBUILD_CFLAGS += $(call cc-disable-warning, unused-const-variable)
 # These result in bogus false positives
 KBUILD_CFLAGS += $(call cc-disable-warning, dangling-pointer)
 
+ifdef CONFIG_LD_IS_LLD
+KBUILD_LDFLAGS += -O2
+endif
+
 ifdef CONFIG_FRAME_POINTER
 KBUILD_CFLAGS	+= -fno-omit-frame-pointer -fno-optimize-sibling-calls
 else