[clang-cl] Add support for /kernel

MSVC defines _KERNEL_MODE when /kernel is passed.
Also, /kernel disables RTTI and C++ exception handling.

https://docs.microsoft.com/en-us/cpp/build/reference/kernel-create-kernel-mode-binary?view=msvc-170

Reviewed By: thakis

Differential Revision: https://reviews.llvm.org/D126719