rustc: disable for ARMV7A with EABI

The Rust project does not support armv7a EABI, only EABIHF [1]. So
disable rustc for this ARM variant.

Fixes:

- http://autobuild.buildroot.net/results/51c98ca5486e7e83725ababa437cbb8204ceb66b/

[1] https://forge.rust-lang.org/platform-support.html



Signed-off-by: Eric Le Bihan <eric.le.bihan.dev@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>