This prevents the linker from discarding any function which uses the
alternative!() macro which would result in runtime relocation crashing.

It is relatively slow without optimizations, but it is still usable.

The UB has been found by rustc's new UB checks with debug assertions
enabled.

This is allowed now that the old physalloc/physfree syscalls have been
removed.

This shrinks the kernel from 905840 bytes to 862408 bytes.