runtime: add the ability to set exterrors in userspace

The UEXTERROR(3) macro is a partial analog to EXTERROR(9) that sets
the current user exterror state and errno. The main difference is
that it

runtime: add the ability to set exterrors in userspace

The UEXTERROR(3) macro is a partial analog to EXTERROR(9) that sets
the current user exterror state and errno. The main difference is
that it returns no value and sets errno directly since that's the
typical pattern in libraries.

While here move the storage and constructor for single-threaded
program's uexterr to its own file.

Reviewed by: kib
Effort: CHERI upstreaming
Sponsored by: Innovate UK
Differential Revision: https://reviews.freebsd.org/D58059

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