Fix and improve usage of atomics

[GaryOderNichts]

• Replaces the usage of the built-in atomics with the OS* atomics from coreinit.
  This avoids a hardware bug described here.

  There appears to be a bug that affects load-exclusive and store-exclusive instructions (an explicit cache flush is required)

  The gcc built-in atomics do not emit a cache flush which might cause issues in the current wut lock implementation.

• Uses OSSwapAtomic in wutstdc++'s once implementation where a compare isn't necessary.

• Uses OSCompareAndSwapAtomicEx to directly update the oldSize in the newlib sbrk implementation.