fortify: Make pointer arguments const

In preparation for using Clang's __pass_object_size attribute, make all
the pointer arguments to the fortified string functions const. Nothing
was changing their values anyway, so this added requirement (needed by
__pass_object_size) requires no code changes and has no impact on
the binary instruction output.

Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Link: https://lore.kernel.org/r/20220208225350.1331628-6-keescook@chromium.org