Compiler Attributes: enable -Wstringop-truncation on W=1 (gcc >= 8)

Commit 217c3e0196 ("disable stringop truncation warnings for now")
disabled -Wstringop-truncation since it was too noisy.

Having __nonstring available allows us to let GCC know that a string
is not meant to be NUL-terminated, which helps suppressing some
-Wstringop-truncation warnings.

Note that using __nonstring actually triggers other warnings
(-Wstringop-overflow, which is on by default) which may be real
problems. Therefore, cleaning up -Wstringop-truncation warnings
also buys us the ability to uncover further potential problems.

To encourage the use of __nonstring, we put the warning back at W=1.
In the future, if we end up with a fairly warning-free tree,
we might want to enable it by default.

Tested-by: Sedat Dilek <sedat.dilek@gmail.com> # on top of v4.19-rc5, clang 7
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Luc Van Oostenryck <luc.vanoostenryck@gmail.com>
Signed-off-by: Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>

scripts/Makefile.extrawarn

@@ -29,6 +29,7 @@ warning-1 += $(call cc-option, -Wmissing-include-dirs)
 warning-1 += $(call cc-option, -Wunused-but-set-variable)
 warning-1 += $(call cc-option, -Wunused-const-variable)
 warning-1 += $(call cc-option, -Wpacked-not-aligned)
+warning-1 += $(call cc-option, -Wstringop-truncation)
 warning-1 += $(call cc-disable-warning, missing-field-initializers)
 warning-1 += $(call cc-disable-warning, sign-compare)