Build LLVM separately for Rust toolchain compilation

The Rust toolchain requires 2 copies of built LLVM when doing
cross-compilation, one for the host and one for the target.

We don't need PGO, LTO, MLGO, etc for the Rust toolchain. We also
don't need clang but it's harder to make the build.py script work
without building clang.

With this, we can build rustc for mac-arm on an intel, though the
following tools fail to build:
- cargo (missing OpenSSL for ARM)
- rust-analyzer (stage2 build is broken with --host X --target X, it
  can't find std)

Bug: 1386212
Change-Id: I7518f3e51c238bc4ca82e63b94e99c1973488e0a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4241203
Reviewed-by: Collin Baker <[email protected]>
Commit-Queue: danakj <[email protected]>
Reviewed-by: Hans Wennborg <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1120216}
NOKEYCHECK=True
GitOrigin-RevId: 873bab1eefc07bd8e1194e8db54b1424818ffa24

Author: danakj