[MachO] Disable atexit()-based lowering when LTO'ing kernel/kext code

The kernel and kext environments do not provide the `__cxa_atexit()`
function, so we can't use it for lowering global module destructors.

Unfortunately, just querying for "compiling for kernel/kext?" in the LTO
pipeline isn't possible (kernel/kext identifier isn't part of the triple
yet) so we need to pass down a CodeGen flag.

rdar://93536111

Differential Revision: https://reviews.llvm.org/D148967