x86: declare halt() as DONT_TRANSLATE for verification

Fix licenses (the rest)

s/D61/DATA61/ in license headers for consistency

trivial: print kernel entry reason on halt

It is generally useful to know what operation was occuring
when the kernel halted, as this is usually called from assert.

Remove many DONT_TRANSLATE markers.

The vast majority of the DONT_TRANSLATE markers in the kernel are used
to hide __asm__ statements and builtin functions
(e.g. __builtin_unreachable ()) from the C-to-Isabelle parser.

The parser now supports underscore identifiers and many __asm__ statements,
and the builtin functions are prototyped, meaning the vast majority of the
DONT_TRANSLATE markers can be dropped. The remaining markers cover functions
that must be treated specially.

x86: replace idle/halt[.S] with idle.c