assert: Mark `__assert_fail` as `cold`

This helps compilers split the codegen for setting up the arguments
(`__expression`, `__filename`, etc...) from the potentially hot cold
where the `assert` is to a presumably cold region on the assertion
failure path.

Reviewed-by: Florian Weimer <fweimer@redhat.com>
Reviewed-by: Sam James <sam@gentoo.org>