Clang requires libatomic to be linked via LDFLAGS

Clang requires the libatomic to be linked via LDFLAGS or else the linker
will fail to recognise __atomic macros.

An error as a result of not passing -latomic to the linker can be seen
below:

ld.lld: error: undefined symbol: __atomic_fetch_add_2
>>> referenced by sds.c:185
>>>               lto.tmp:(sdsdupshared)
clang-10: error: linker command failed with exit code 1 (use -v to see invocation)
make[1]: *** [Makefile:276: keydb-server] Error 1
Former-commit-id: dcd7b684bbb7b4719a0e44d5764b01c48eb619dd

src/Makefile

@@ -70,6 +70,7 @@ endif
 
 ifeq ($(COMPILER_NAME),clang)
 	CXXFLAGS+= -stdlib=libc++ 
+	LDFLAGS+= -latomic
 endif
 
 # To get ARM stack traces if Redis crashes we need a special C flag.