ANDROID: rust: use target.json for aarch64

Enabling Rust currently prints the following warning to the log:

	warning: unknown feature specified for `-Ctarget-feature`: `reserve-x18`
	  |
	  = note: it is still passed through to the codegen backend
	  = help: consider filing a feature request

The best way to fix this warning would be to backport the relevant
one-line change in the Rust compiler to our 1.73.0 compiler and make a
respin. However, as a temporary measure until such a respin happens, we
can silence the warning by using a target.json for aarch64.

Bug: 348496478
Change-Id: I3fce783d6f7be1c72d00f8b59fb248a78bfefaae
Signed-off-by: Alice Ryhl <aliceryhl@google.com>