rust: compile host package per target

rust: compile host package per target

While the compiled binaries are intended to run on the host system, the
rust/host package does include the target matching the configured
OpenWrt target.

If using (for example) ./scripts/env to switch between different
OpenWrt configurations, this will cause issues if the different
configuration is for a different target. In such case there will be a
mismatch between the available Rust target and OpenWrt target and the
following error will be printed:

> error[E0463]: can't find crate for `core`
> note: the `XXX` target may not be installed

This fix will add the RUSTC_TARGET_ARCH as HOST_BUILD_DIR and CARGO_HOME
suffix, such that rust/host will be compiled in case an OpenWrt
configuration change causes the RUSTC_TARGET_ARCH to change.

Fixes: #21530
Signed-off-by: Orne Brocaar <info@brocaar.com>
[Applied Jeffery To's suggestion for build and install path]
Signed-off-by: Tianling Shen <cnsztl@immortalwrt.org>