Asserted that the Cortex-R52 port refuses the options it documents as refused (#687)

ports/cortex_r52/gnu/CMakeLists.txt rejects two option combinations at
configure time -- TX_R52_ENABLE_VFP without TX_R52_FLOAT_ABI=hard, and
TX_R52_ENABLE_FIQ_NESTING without TX_R52_ENABLE_FIQ.  Nothing exercised
either.  A guard that has stopped firing looks exactly like a guard nobody
has tripped, so both could have been silently disabled by a typo in a
variable name at any point and no check would have noticed.

check_gcc.sh gains a sixth stage that configures each rejected combination
and asserts the configure fails.  It needs no new harness: the script already
runs cmake as a subprocess for the CMake example builds, and the port's
CMakeLists is included by the toolchain file alone, so no example flags are
needed and the two negative configures stop almost immediately.

Two things the stage does that a thinner version would not.

It asserts the message text, not just the exit status.  A configure that
fails for an unrelated reason would otherwise be recorded as a guard doing
its job.

It also configures the SUPPORTED combination and requires that to succeed.
Two negative assertions on their own are satisfied by a guard that rejects
everything -- the port would be unbuildable and the check would still pass.
The positive case is what separates a guard that works from one that is
merely always on.

Verified negatively, four deliberate breaks, each caught:

  VFP guard condition forced false      -> "configure succeeded, but this
                                           combination cannot build"
  FIQ nesting guard forced false        -> the same, on that case
  guard fires, message text changed     -> "configure failed, but not on the
                                           expected guard"
  VFP guard condition forced true       -> "the supported combination was
                                           refused"

The fourth is the one the positive case exists for and the only one a
refusals-only stage would have missed.  ports/cortex_r52/gnu/CMakeLists.txt
was confirmed byte-identical to dev afterwards.

Full run passes: exit 0, all six stages, and --asm-only correctly skips the
new one.

Assisted-by: Claude Code (Opus 5) <[email protected]>

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