__sys builtin support for AArch64 (#146456)

Adds support for __sys Clang builtin for AArch64

__sys is a long existing MSVC intrinsic used to manage caches, tlbs, etc
by writing to system registers:
* It takes a macro-generated constant and uses it to form the AArch64 SYS instruction which is MSR with op0=1. The macro drops op0 and expects the implementation to hardcode it to 1 in the encoding.
* Volume use is in systems code (kernels, hypervisors, boot environments, firmware)
* Has an unused return value due to MSVC cut/paste error

Implementation:
* Clang builtin, sharing code with Read/WriteStatusReg
* Hardcodes the op0=1
* Explicitly returns 0
* Code-format change from clang-format
* Unittests included
* Not limited to MSVC-environment as its generally useful and neutral

0 files changed, 0 insertions, 0 deletions