package/rustc: RUSTC_TARGET_NAME is needed to build host-rust

As reported on bugzilla [1], the host-rust package fail when
the target architecture or the target libc is not supported.

The error is the following:
failed to parse TOML configuration 'config.toml': expected a table key, found a right bracket at line 15

In such case BR2_PACKAGE_HOST_RUSTC_TARGET_ARCH_SUPPORTS is
not set thus RUSTC_TARGET_NAME is also not set [2].

But RUSTC_TARGET_NAME is needed to generate the file config.toml [3]

Add BR2_PACKAGE_HOST_RUSTC_TARGET_ARCH_SUPPORTS in the host-rust
dependency.

The commit [4] that allowed to select host-rust when the target
architecture or the target libc is not supported, should have
allowed to select only host-rustc-bin.

Fixes:
Bug #12691

[1] https://bugs.busybox.net/show_bug.cgi?id=12691
[2] https://git.buildroot.net/buildroot/tree/package/rustc/rustc.mk?h=2020.05-rc1#n10
[3] https://git.buildroot.net/buildroot/tree/package/rust/rust.mk?h=2020.05-rc1#n41


[4] 025b863e

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Cc: Sam Voss <sam.voss@gmail.com>
Reviewed-by: Sam Voss <sam.voss@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>