toolchain: move iamcu output format/arch to SoC

Instead of defining the output format in the toolchain environment
setup, define this per SoC that supports the ABI.

This will allow us to directly referencing an installed toolchain
without the need for declaring ZEPHYR_GCC_VARIANT

Change-Id: Icd1a7a21acbf075a51854fb2ebb3ef06788ce9b5
Signed-off-by: Anas Nashif <anas.nashif@intel.com>