concurrent-ruby-ext: fix build on Darwin 32-bit

[barracuda156]

Fixes: #1063

[barracuda156]

For the record, this works neatly with Ruby 3.3, while Ruby 3.2 also needs -Wno-incompatible-pointer-types to be passed, otherwise this fails:

atomic_reference.c: In function 'ir_compare_and_set':
atomic_reference.c:83:57: error: passing argument 3 of 'OSAtomicCompareAndSwap32' from incompatible pointer type [-Wincompatible-pointer-types]
   83 |   if (OSAtomicCompareAndSwap32(expect_value, new_value, &DATA_PTR(self))) {
In file included from atomic_reference.h:9,
                 from atomic_reference.c:20:
/usr/include/libkern/OSAtomic.h:132:93: note: expected 'volatile int32_t *' {aka 'volatile int *'} but argument is of type 'void **'
  132 | bool    OSAtomicCompareAndSwap32( int32_t __oldValue, int32_t __newValue, volatile int32_t *__theValue );
      |                                                                           ~~~~~~~~~~~~~~~~~~^~~~~~~~~~
make: *** [atomic_reference.o] Error 1

make failed, exit code 2

[eregon]

Arguments should be explicitly casted to avoid the warning/error.

One problem though is we have no way to test macOS 32-bit, so it should be considered basically unsupported.

[barracuda156]

@eregon If you suggest a better fix, I can test that locally both on ppc and i386.