internal: use loop {} to produce never value

[nbdd0121]

In the init!/pin_init! macros, we rely on a trick that assigns never
(!) values to all mentioned fields in never-executed code to let the
compiler check that all fields have been initialized.

Currently we use ::core::panic!() to produce this value, but before Rust
1.91.0, it creates outlined panic_cold_explicit functions which do not
get removed by the optimizer, thus leaving dead code behind in the binary.
This has been fixed by [1], which lands in Rust 1.91.0+, higher than the
kernel minimum version 1.85.0.

This causes ~200 dead panic_cold_explicit instances being included in the
binary, with ~90 of them from nova-core's usage of pin-init.

Work around the issue by using loop {} which creates the never value
without macro expansion or function call at all. All instances of
panic_cold_explicit outside libcore are removed by this change in my
kernel build.