package/rustc: add support for Tier 1 and Tier 2 platforms

This commit reworks how BR2_PACKAGE_HOST_RUSTC_TARGET_ARCH_SUPPORTS is
defined to more clearly map with the list of platforms supported by
Rust as listed at
https://doc.rust-lang.org/nightly/rustc/platform-support.html. Indeed,
the situation is not as simple as a list of architectures, all
supported for both glibc and musl.

So instead, we take the approach of directly mapping with what's
described at
https://doc.rust-lang.org/nightly/rustc/platform-support.html, which
means:

 * A list of Tier 1 platforms (in fact just 3 platforms)

 * A list of Tier 2 platforms with host tools (i.e where rustc and
   cargo themselves are available for the target, something that isn't
   relevant for Buildroot)

 * A list of Tier 2 platforms with no host tools support.

For each platform, we add as a comment its Rust tuple name, as listed
at https://doc.rust-lang.org/nightly/rustc/platform-support.html

, and
then the corresponding Buildroot architecture/libc dependency.

This is obviously more verbose than it was, but it is also a lot
easier to maintain.

With this, a total of 16 new platforms are supported, 13 of which are
musl-based. The additional non-musl platforms are ARMv5TE, RISC-V
64-bit and Sparc64.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>