Real-Time Executive for Multiprocessing Systems

bsps/mips: Emulate rdhwr for the thread pointer
The compiler emits rdhwr  $3,$ 29 to read the thread pointer for every
access to a thread-local object.  It does this on every MIPS target,
because the instruction exists since MIPS32r2 and the operating system
is expected to emulate it where the processor lacks it.  The R3900 of
mips/jmr3904 lacks it, so thread-local storage did not work at all.
Emulate the instruction in the reserved instruction exception, which is
the only way the processor reports it.  A processor which has the
instruction never raises the exception, so the emulation costs nothing
there.
An instruction in a branch delay slot is left to the fatal handler: the
exception program counter references the branch, so the emulation cannot
resume by advancing it.
Close #5689.
Assisted-by: Claude:claude-opus-5 claude-code
Signed-off-by: Sebastian Huber sebastian.huber@embedded-brains.de