runtime: add uexterr_set(3) to set errors in userspace

[brooks]

Summary

XXX: this lacks documentation and probably isn't the right interface,
but it's enough to add tests for new error formats without disturbing
the kernel. This is mostly to spark conversation.

Effort: CHERI upstreaming
Sponsored by: Innovate UK

[brooks]

The big thing that needs to be dealt with for this to make sense is to have a way to specify the filename that isn't a table embedded in libc. A first cut could be a special index that means no-file and perhaps using -1 as the line, but without that, we can't scale beyond files in the base system. (This also poses a problem for out of tree kernel modules.) We might also want macros like the kernel, but I'm less sure of that.

[kib]

lib/libthr/thread/thr_syscalls.c

664

Create a libc helper in the private version namespace that takes uexterrp, and call it there and in the libc' uexterr_set().