BACKPORT: usercopy: fold builtin_const check into inline function

Instead of having each caller of check_object_size() need to remember to
check for a const size parameter, move the check into check_object_size()
itself. This actually matches the original implementation in PaX, though
this commit cleans up the now-redundant builtin_const() calls in the
various architectures.

Signed-off-by: Kees Cook <keescook@chromium.org>

Change-Id: I348809399c10ffa051251866063be674d064b9ff
(cherry picked from 81409e9e)
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>