Fix link-time optimization to work correctly for unit tests (i.e. -flto flag) (#1290) (#1296)

* We compile various c files into object and package them into library
(.a file) using ar to feed to unit tests. With new GCC versions, the
objects inside such library don't participate in LTO process without
additional flags.
* Here is a direct quote from gcc documentation explaining this issue:
"If you are not using a linker with plugin support and/or do not enable
the linker plugin, then the objects inside libfoo.a are extracted and
linked as usual, but they do not participate in the LTO optimization
process. In order to make a static library suitable for both LTO
optimization and usual linkage, compile its object files with
-flto-ffat-lto-objects."
* Read full documentation about -flto at
https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html
* Without this additional flag, I get following errors while executing
"make test-unit". With this change, those errors go away.

```
ARCHIVE libvalkey.a
ar: threads_mngr.o: plugin needed to handle lto object
...
..
.
/tmp/ccDYbMXL.ltrans0.ltrans.o: In function `dictClear':
/local/workplace/elasticache/valkey/src/unit/../dict.c:776: undefined
reference to `valkey_free'
/local/workplace/elasticache/valkey/src/unit/../dict.c:770: undefined
reference to `valkey_free'
/tmp/ccDYbMXL.ltrans0.ltrans.o: In function `dictGetVal':
```

Fixes #1290

---------

Signed-off-by: Parth Patel <661497+parthpatel@users.noreply.github.com>

src/Makefile

@@ -25,7 +25,7 @@ ifeq ($(OPTIMIZATION),-O3)
 	ifeq (clang,$(CLANG))
 		OPTIMIZATION+=-flto
 	else
-		OPTIMIZATION+=-flto=auto
+		OPTIMIZATION+=-flto=auto -ffat-lto-objects
 	endif
 endif
 ifneq ($(OPTIMIZATION),-O0)