build: Improve usage of the ISSM toolchain

With this fix now we can use the ISSM standlone toolchain [1] through
ZEPHYR_GCC_VARIANT=issm and ISSM_INSTALLATION_PATH=/path/to/toolchain
variables only, without the need of setting IA_VERSION and ARC_VERSION
always. These are always the same for a given toolchain release.

This is also a way for us to document the currently supported (and
tested) toolchain version.

Tested on Linux only.

[1]
https://software.intel.com/en-us/articles/issm-toolchain-only-download



Change-Id: I6fba8ea9564b2080bd73b627bc7150863401f18d
Signed-off-by: Jesus Sanchez-Palencia <jesus.sanchez-palencia@intel.com>