Core parking broken with NR_CPUS=1

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

While testing the rebuilt debian unstable container, I found:

common/core_parking.c: In function 'core_parking_remove':
common/core_parking.c:230:53: error: array subscript 1 is above array
bounds of 'unsigned int[1]' [-Werror=array-bounds]
  230 |         core_parking_cpunum[i] = core_parking_cpunum[i + 1];
      |                                  ~~~~~~~~~~~~~~~~~~~^~~~~~~
common/core_parking.c:31:21: note: while referencing 'core_parking_cpunum'
   31 | static unsigned int core_parking_cpunum[NR_CPUS] = {[0 ...
NR_CPUS-1] = -1};
      |                     ^~~~~~~~~~~~~~~~~~~


which is an legitimate complaint - this logic is simply broken with
NR_CPUS=1.

In principle, I think the best fix is probably to have CORE_PARKING
depend on NR_CPUS > 1, except none of the interacting x86 code has been
written in a way that would be compatible.

But it also occurs to me that this is the kind of thing an embedded x86
usecase would want to compile.  Frankly, its niche even on regular x86
use-cases.

Except having looked at the code, it's a different to the other thing we
call core parking which is the smt=0 logic to keep the stacks valid for
cores/threads we don't want to use, because of #MC broadcast problems -
and this logic does need to stay.

Thoughts?

~Andrew