Allow passing RTLIB=compiler-rt to make

Not all linux users will have libatomic installed.
When using clang as the system compiler with compiler-rt as the default
runtime library instead of libgcc, atomic builtins may be provided by compiler-rt.
This change allows such users to pass RTLIB=compiler-rt to make sure
the build doesn't error out on the missing (unnecessary) libatomic.

closes https://github.com/official-stockfish/Stockfish/pull/3597

No functional change

src/Makefile

@@ -386,10 +386,12 @@ ifeq ($(COMP),clang)
 	ifneq ($(KERNEL),Darwin)
 	ifneq ($(KERNEL),OpenBSD)
 	ifneq ($(KERNEL),FreeBSD)
+	ifneq ($(RTLIB),compiler-rt)
 		LDFLAGS += -latomic
 	endif
 	endif
 	endif
+	endif
 
 	ifeq ($(arch),$(filter $(arch),armv7 armv8))
 		ifeq ($(OS),Android)