xtensa: add config for toolchain version

Use TOOLCHAIN_VARIANT to indicate what XCC toolchain release
to use.

Set a reasonable default for the RG-2016.4 toolchain release.
D_108mini, D_212GP, D_233L are only in RF-2016.4, set that
appropriately.

Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>