Add support for building Rust apps on the NuttX simulator running on
Intel (x86_64) macOS.
- Add tools/x86_64-unknown-nuttx-macho.json, a Mach-O Rust target for
the macOS sim.
- Rust.defs: select the new x86_64 macho target for sim on macOS.
- boards/sim/.../Make.defs: keep the default PIC/PIE-compatible code
generation on macOS by not adding -fno-pic/-mcmodel=medium.
- sethost.sh: enable CONFIG_HOST_X86_64 for non-arm64 hosts.
Signed-off-by: Shoji Tokunaga <toku@mac.com>
