rust-common: Set llvm-target correctly for cross SDK targets

rust-common: Set llvm-target correctly for cross SDK targets

When a 'BUILD' target is requested we shouldn't be looking at TARGET_SYS but
at BUILD_SYS. Due to the way rust mangles triplets, we need the HOST_SYS triplet
to work with existing code - fixing that issue is a separate patch.

Also drop the arch_abi argument, it doens't make any sense to a getVar() call
and was a copy and paste error.

Based on a patch from Otavio Salvador <otavio@ossystems.com.br> but separated out
and tweaked.

Fixes: bd36593ba3 ("rust-common: Drop LLVM_TARGET and simplify")
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>