Makefile: add -fno-builtin-stpcpy

LLVM implemented a recent "libcall optimization" that lowers calls to
`sprintf(dest, "%s", str)` where the return value is used to
`stpcpy(dest, str) - dest`. This generally avoids the machinery involved
in parsing format strings. This optimization was introduced into
clang-12. Because the kernel does not provide an implementation of
stpcpy, we observe linkage failures for almost all targets when building
with ToT clang.

The interface is unsafe as it does not perform any bounds checking.
Disable this "libcall optimization" via `-fno-builtin-stpcpy`.

Unlike
commit 5f074f3 ("lib/string.c: implement a basic bcmp")
which cited failures with `-fno-builtin-*` flags being retained in LLVM
LTO, that bug seems to have been fixed by
https://reviews.llvm.org/D71193, so the above sha can now be reverted in
favor of `-fno-builtin-bcmp`.

Reported-by: Sami Tolvanen <samitolvanen@google.com>
Suggested-by: Dávid Bolvanský <david.bolvansky@gmail.com>
Suggested-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
Cc: stable@vger.kernel.org # 4.4
Link: https://bugs.llvm.org/show_bug.cgi?id=47162
Link: ClangBuiltLinux/linux#1126
Link: https://reviews.llvm.org/D85963

Makefile

@@ -698,6 +698,12 @@ KBUILD_CFLAGS += $(call cc-ifversion, -lt, 0409, \
 # Tell gcc to never replace conditional load with a non-conditional one
 KBUILD_CFLAGS	+= $(call cc-option,--param=allow-store-data-races=0)
 
+# The compiler may "libcall optimize" certain function calls into the below
+# functions, for architectures that don't use -ffreestanding. If we don't plan
+# to provide implementations of these routines, then prevent the compiler from
+# emitting calls to what will be undefined symbols.
+KBUILD_CFLAGS	+= -fno-builtin-stpcpy
+
 # check for 'asm goto'
 ifeq ($(call shell-cached,$(CONFIG_SHELL) $(srctree)/scripts/gcc-goto.sh $(CC) $(KBUILD_CFLAGS)), y)
 	KBUILD_CFLAGS += -DCC_HAVE_ASM_GOTO