kbuild: prevent CC_FLAGS_LTO self-bloating on recursive rebuilds

CC_FLAGS_LTO gets initialized only via +=, never with := or =.
When building with CONFIG_TRIM_UNUSED_KSYMS, Kbuild may perform
several kernel rebuilds to satisfy symbol dependencies. In this
case, value of CC_FLAGS_LTO is concatenated each time, which
triggers a full rebuild.
Initialize it with := to fix this.

Fixes: dc5723b0 ("kbuild: add support for Clang LTO")
Signed-off-by: Alexander Lobakin <alobakin@pm.me>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20210121184544.659998-1-alobakin@pm.me

Makefile

@@ -895,10 +895,10 @@ endif
 
 ifdef CONFIG_LTO_CLANG
 ifdef CONFIG_LTO_CLANG_THIN
-CC_FLAGS_LTO	+= -flto=thin -fsplit-lto-unit
+CC_FLAGS_LTO	:= -flto=thin -fsplit-lto-unit
 KBUILD_LDFLAGS	+= --thinlto-cache-dir=$(extmod-prefix).thinlto-cache
 else
-CC_FLAGS_LTO	+= -flto
+CC_FLAGS_LTO	:= -flto
 endif
 CC_FLAGS_LTO	+= -fvisibility=hidden