Fix GCC 12 infinite loop in strlen

Strlen was jumping to strlen as part of -ftree-loop-distribute-patterns.
Seems this sort of issue can be common and generally fixed with
-fno-builtin. However, some of those optimizations may be useful and I
don't think our semantics for the functions are wildly different than
the standard, so I'm fixing this surgically with
-ftree-loop-distribute-patterns to just util.c. That may be the wrong
call, so if this bites us again, let's go more aggressive. Fixes #660.

Below are some filesizes for stm32f105/prod/floppy/target.bin with
different compilers/branches that I trialed to get an idea of the impact
of these optimizations.

stable-v3:
GCC 11: 83180 baseline
GCC 11: 83180 util.c no-tree-loop-distribute-pattern
GCC 11: 83180 util.c no-builtin
GCC 11: 83200 all    no-tree-loop-distribute-pattern
GCC 11: 83336 all    no-builtin

GCC 12: 83500 baseline (broken)
GCC 12: 83512 util.c no-tree-loop-distribute-pattern
GCC 12: 83512 util.c no-builtin
GCC 12: 83524 all    no-tree-loop-distribute-pattern
GCC 12: 83544 all    no-builtin

master:
GCC 12: 89996 baseline (broken)
GCC 12: 90008 util.c no-tree-loop-distribute-pattern
GCC 12: 90020 util.c no-builtin
GCC 12: 90020 all    no-tree-loop-distribute-patterns
GCC 12: 90064 all    no-builtin

Author: ejona86

src/Makefile

@@ -52,6 +52,10 @@ endif
 .PHONY: $(SRCDIR)/build_info.c
 build_info.o: CFLAGS += -DFW_VER="\"$(FW_VER)\""
 
+# Avoid infinite loops due to GCC noticing code that can be replaced by a call
+# to a standard library function... within our implementation of that function.
+util.o: CFLAGS += -fno-tree-loop-distribute-patterns
+
 ff_cfg_defaults.h: $(ROOT)/examples/FF.CFG
 	$(PYTHON) $(ROOT)/scripts/mk_config.py $< $@