Implement CET Shadow Stack (Intel Controlflow Enforcement Technology) support on...

Implement CET Shadow Stack (Intel Controlflow Enforcement Technology) support on Windows

Patch by Petr Penzin.

Windows support for CET is limited to shadow stack, which is enabled
by setting a PE bit in the linker.

Docs:

MSVC linker flag:
https://docs.microsoft.com/en-us/cpp/build/reference/cetcompat?view=vs-2019

IMAGE_DLLCHARACTERISTICS_EX_CET_COMPAT PE bit:
https://docs.microsoft.com/en-us/windows/win32/debug/pe-format#extended-dll-characteristics

Differential Revision: https://reviews.llvm.org/D70606