Host builtin support, by backend

Generated by `scripts/host_matrix.sh`. Do not edit.

Which backends have a lowering for each host / IO builtin, asked of the compiler rather than remembered. refused means the backend says so itself, which is the correct way to lack something; MISSING means a program using it fails with unbound variable, blaming the user for a backend hole; bare means the backend has it on -rv --bare only, where the program is handed the machine instead of a host; stub means it compiles there and then ABORTS AT RUNTIME, which is what the -rv prelude's float shims do while contrib/softfloat is not wired in -- yes would be the flattering lie and refused the other one; unattributed means the backend refused but named a different builtin, so the probe never reached its subject and the cell is evidence about that other name.

The interpreter has every one of these, and is not a column.

RV32I is the fifth backend and was missing from this table until v0.1.332 — a record whose job is to say which backend has which builtin, with a backend not in it. Most of its column is refused, which is the right answer for an integer-subset target that hands the program a machine rather than a host.