cmake: xtools: use SoC name in path to xtensa toolchain

Xtensa requires building a new toolchain for a specific SoC.
By default xtools built Xtensa toolchains all have prefix of
xtensa-zephyr-elf. In order to distinguish different toolchains,
they are now placed in their own directories under their SoC
name. This allows us to have multiple Xtensa toolchains
targeting multiple SoCs.

The additional level in path name is introduced in SDK v0.11
and sdk-ng master.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>