rustc: use RUSTC_{HOST,TARGET}_NAME

utils/check-package complains as follows:

  package/rustc/rustc.mk:10: possible typo: RUST_TARGET_NAME -> *RUSTC*
  package/rustc/rustc.mk:18: possible typo: RUST_HOST_NAME -> *RUSTC*

As RUST_{HOST,TARGET}_NAME are related to the Rust compiler, it
sounds sensible to rename them to RUSTC_{HOST,TARGET}_NAME.

So update all rust related packages to use the new variables.

Signed-off-by: Eric Le Bihan <eric.le.bihan.dev@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>