rust: modify Rust packaging to be usable as host-tool only

This commit modifies the host-rust virtual package to default to
host-rust-bin when no other selection has been made, as long as the
host supports rust. This allows host only tools to still use rust when
the target architecture does not support it.

Add target-specific variable which is used to differentiate host and
target arch requirements (BR2_PACKAGE_HOST_RUSTC_TARGET_ARCH_SUPPORTS).

A target package shall depend on this variable where a host package will
use the previously defined BR2_PACKAGE_HOST_RUSTC_ARCH_SUPPORTS. The new
"target" version is selectable for the same set of architectures as
before, but now depends on the host variant.

Signed-off-by: Sam Voss <sam.voss@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Reviewed-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Tested-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>