init: add `SYS_INIT_NAMED`

Add a variant of `SYS_INIT` that is explicitly given a name, instead of
using the function name. This allows multiple `SYS_INIT` entries to use
the same initialisation function, without needing the `__COUNTER__`
hack in `Z_SYS_NAME`.

The `__COUNTER__` hack is undesirable as it makes it impossible to
reconstruct the name of the `struct init_entry` instance from the macro
parameters.

Signed-off-by: Jordan Yates <jordan.yates@data61.csiro.au>