[lavapipe] Add temp AArch64 JIT target overrides

Prebuilt LLVM headers for Fuchsia ARM64 default to x86_64, causing
LLVM JIT to emit x86_64 code and crash on ARM64 devices while
running lavapipe.

As a temporary fix until prebuilts are corrected, explicitly init
AArch64 targets and set JIT triples to aarch64-unknown-fuchsia on
AArch64 builds.

Bug: 541277546, 339107217
Change-Id: I0adbeab26c7d06acf0c271e5316ae485659da0c3
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/mesa/+/1736269
Reviewed-by: Emircan Uysaler <emircan@google.com>
Fuchsia-Auto-Submit: Emircan Uysaler <emircan@google.com>
Reviewed-by: David Gilhooley <dgilhooley@google.com>
Commit-Queue: Emircan Uysaler <emircan@google.com>
Reviewed-by: Josh Gargus <jjosh@google.com>