Adding Int128 and UInt128 with a base software implementation (#69204)

Adding Int128 and UInt128 with a base software implementation (#69204)

* Adding barebones Int128 and UInt128 structs

* Special case Int128 and UInt128 alignment on x64 Unix and Arm64

* Implementing Int128 and UInt128

* Adding tests for Int128 and UInt128

* Updating Int128/UInt128 to respect the System V ABI ordering

* Fixing an issue with UInt128->BigInteger setting the wrong sign

* Don't use Unsafe.As in the Int128/UInt128 hex parsing logic

* Adding Int128 P/Invoke tests and ensure R2R correctly sets the packing

* Fixing some issues with the Int128 interop test for non-Windows

* Ensure that floating-point conversions exist for Int128 and UInt128

* Fixing the casing of a couple fields

* Revert "Don't use Unsafe.As in the Int128/UInt128 hex parsing logic"

This reverts commit 09e8bfc52d5b1b7f7b341f318bf1046b19768dd3.

* Adjusting the Int128/UInt128 generic math tests to have consistent ordering

* Responding to PR feedback

* Ensure that pNativeLayoutInfo alignment is initialized for Int128/UInt128

* Don't use Unsafe.As in the Int128/UInt128 hex parsing logic

* Skip the Interop/PInvoke/Int128 tests on Mono