Add a custom aarch64 Mach-O Rust target for macOS sim builds and use it
instead of an Apple Darwin Rust target. This keeps Rust cfg values aligned
with NuttX while producing Mach-O objects required by the simulator link.
Also align sim host handling for aarch64 Linux by detecting `aarch64` as
`HOST_ARM64` and avoiding x86-specific `-mcmodel` and `-no-pie` options
on ARM64 hosts.
Signed-off-by: Shoji Tokunaga <toku@mac.com>
7c38d58675