BACKPORT: FROMLIST: 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.

(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>

Makefile

@@ -788,6 +788,7 @@ LDFLAGS += --lto-O2
 else
 LDFLAGS += -O2
 endif
+LDFLAGS += --lto-O2
 endif
 
 # Tell gcc to never replace conditional load with a non-conditional one