rust-common/rust-cross: Clean up target json generation code

rust-common/rust-cross: Clean up target json generation code

Some of the subtleties in the different codepaths for target rust json generation
were not easy to spot. Start to simplfy the code to make this clearer.

This patch should not have any functionality change although ABIEXTENSION
has to be excluded from the function signature, the triplet would normally
cover anything set there.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>