RTEMS | spmisc01: Avoid use of unreachable keyword (#5261)

[Joel Sherrill (@joel)]

Joel Sherrill created an issue: https://gitlab.rtems.org/rtems/rtos/rtems/-/issues/5261

Assignee: Joel Sherrill

## Summary
For C23 and later, [_unreachable_](https://en.cppreference.com/w/c/program/unreachable) is now a keyword. Any uses in source should be changed.

C23 is the default in GCC 15.

## Steps to reproduce

### Pre-set options

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