[compiler-rt][Darwin] Fix GetOSMajorKernelOffset() on watchOS

`TARGET_OS_IOS` and `TARGET_OS_WATCH` are not mutually exclusive.
`SANITIZER_IOS` is defined for all embedded platforms.  So the branch
for watchOS is never taken.  We could fix this by switching the order
of the branches (but the reason for doing so is non-obvious).  Instead,
lets use the Darwin-specific `TARGET_OS_*` macros which are mutually
exclusive.