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`.

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>
Reviewed-by: Kees Cook <keescook@chromium.org>
Cc: stable@vger.kernel.org # 4.4
Link: https://bugs.llvm.org/show_bug.cgi?id=47162
Link: ClangBuiltLinux#1126
Link: https://reviews.llvm.org/D85963

Makefile

@@ -578,6 +578,7 @@ ifneq ($(LLVM_IAS),1)
 CLANG_FLAGS	+= -no-integrated-as
 endif
 CLANG_FLAGS	+= -Werror=unknown-warning-option
+CLANG_FLAGS	+= -fno-builtin-stpcpy
 KBUILD_CFLAGS	+= $(CLANG_FLAGS)
 KBUILD_AFLAGS	+= $(CLANG_FLAGS)
 export CLANG_FLAGS