Log Message:
lang/rust: Default to RUST_TYPE=bin on earm

As discussed on tech-pkg@, rust does not build on earm, even if one
has much more memory than the typical 1GB.  For earm, default to
RUST_TYPE bin, so that straightforward use of pkgsrc leads to working
programs, at the expense of our goal of providing pkgsrc rather than
pkgbin.

Test NATIVE_MACHINE_PLATFORM, so that e.g. crossbuilding for earm from
x86_64 should still build from source.

Use ?=, so that people with an explicit RUST_TYPE=src will have that
honored (and then fail, but presumably that's what they want, or they
are trying to fix rust to build without such enormous memory use).

On NetBSD 9 RPI3, 'make package' in libimagequant correctly depends on
rust-bin.