[LFI] Introduce AArch64 LFI Target (#167061)

This PR is the first step towards introducing LFI into LLVM as a new
sub-architecture backend of AArch64. For details, please see the
[RFC](https://discourse.llvm.org/t/rfc-lightweight-fault-isolation-lfi-efficient-native-code-sandboxing-upstream-lfi-target-and-compiler-changes/88380),
which has been approved for AArch64.

This patch creates the `aarch64_lfi` architecture, and marks the
appropriate registers as reserved when it is targeted (`x25`, `x26`,
`x27`, `x28`). It also adds a Clang driver toolchain for targeting LFI,
and updates the compiler-rt CMake to allow builds for the `aarch64_lfi`
target. The patch also includes documentation for LFI and the rewrites
that will be implemented in future patches.

I am planning to split the relevant modifications for LFI into a series
of patches, organized as described below (after this one). Please let me
know if you'd like me to split the changes in a different way, or
provide one big patch.

1. The next patch will introduce the `MCLFIExpander` mechanism for
applying the MC-level rewrites needed by LFI, along with the
`.lfi_expand` and `.lfi_no_expand` assembly directives when targeting
LFI. A preview can be seen on the `lfi-project`
[fork](https://github.com/llvm/llvm-project/compare/main...lfi-project:llvm-project:lfi-patchset/aarch64-pr-2).

2. The following patch will create an `MCLFIExpander` for the AArch64
backend that performs LFI expansions. This patch will contain the
majority of the LFI-specific logic.

3. The final patch will add an optimization to the rewriter that can
eliminate redundant guard instructions that occur within the same basic
block.

We plan to introduce x86-64 support after further discussion and once
the `MCLFIExpander` infrastructure is in place.

Please let me know your feedback, and thank you very much for your help
and guidance in the review process.