Move the NuttX-specific Rust target specification files from apps/tools
to nuttx/tools so files with the same purpose are kept in one place.
The existing aarch64 Mach-O target spec for macOS simulator builds
already lives under nuttx/tools. Keeping the remaining NuttX Rust target
specs there as well makes the layout consistent and lets both Make and
CMake builds refer to target specs from the NuttX tree.
Signed-off-by: Shoji Tokunaga <toku@mac.com>
