libcpu: Remove false positive null ptr deref warning for GCC -O3

libcpu: Remove false positive null ptr deref warning for GCC -O3

i386_lex_CFLAGS is a make variable to tweak the i386_lex.c.o
specific compile flags. The are already some -W defined there, I'd
like to add one to avoid seeing false warning during build with -O3
compile flag.

GCC 14.2 (as well as 13.x) produce a false positive warning for the
i386 lexer, a bug is filled already against GCC for that, CLANG is
immune.

* libcpu/Makefile.am
i386_lex_CFLAGS += $(subst -O3,-Wno-null-dereference,$(findstring -O3,$(CFLAGS)))

https://sourceware.org/bugzilla/show_bug.cgi?id=32527

Signed-off-by: Philippe Benard <phi.debian@gmail.com>