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Control-Flow Enforcement Technology - Shadow Stack support (LLVM side)

Description

Control-Flow Enforcement Technology - Shadow Stack support (LLVM side)

Shadow stack solution introduces a new stack for return addresses only.
The HW has a Shadow Stack Pointer (SSP) that points to the next return address.
If we return to a different address, an exception is triggered.
The shadow stack is managed using a series of intrinsics that are introduced in this patch as well as the new register (SSP).
The intrinsics are mapped to new instruction set that implements CET mechanism.

The patch also includes initial infrastructure support for IBT.

For more information, please see the following:
https://software.intel.com/sites/default/files/managed/4d/2a/control-flow-enforcement-technology-preview.pdf

Differential Revision: https://reviews.llvm.org/D40223

Change-Id: I4daa1f27e88176be79a4ac3b4cd26a459e88fed4