Merge branch '2023-11-10-improve-semihosting-armv6' into next

To quote the author:
This series has a few fixes for semihosting on ARMv6 and older CPUs. The
first two patches address problems regarding the stack pointer and link
register. U-Boot runs in supervisor mode, so taking a software interrupt
will clobber sp/lr. I think we really should run in system mode, since
it has separate sp/lr registers. To quote ARM DDI 0100I:

> The remaining mode is System mode, which is not entered by any
> exception and has exactly the same registers available as User mode.
> However, it is a privileged mode and is therefore not subject to the
> User mode restrictions. It is intended for use by operating system
> tasks that need access to system resources, but wish to avoid using
> the additional registers associated with the exception modes. Avoiding
> such use ensures that the task state is not corrupted by the
> occurrence of any exception.

However, the processor mode has been supervisor for such a long time
(since relocation got introduced) that I would rather not touch it.