First, this calling convention doesn't actually exist in OR1K
and trying to use it in Asserts build causes an UNREACHABLE.

Second, I tried to introduce it and it does not appear to produce
any measurable benefit: not only OR1K has a ton of CSRs but also
it is quite hard, if not realistically impossible, to produce
the kind of register pressure that would be relieved by sparing
a few more CSRs for our exception raising function calls, since
temporaries don't have to be preserved before a noreturn call
and spilling over ten registers across an exceptional edge
is not something that the code we care about would do.

Third, it produces measurable drawbacks: it inflates code size
of check:* functions by adding spills. Of course, this could be
alleviated by making __artiq_raise coldcc as well, but what's
the point anyway?