We can't have a shebang and of course all variables are unused.

Signed-off-by: Marc Herbert <marc.herbert@intel.com>

"COMPILER" is a very common name with a high risk of collision. Don't
use it for a minor, local variable. This also reduces confusion.

Signed-off-by: Marc Herbert <marc.herbert@intel.com>

If the user requested the Cadence toolchain by defining
$XTENSA_TOOLS_ROOT, then do not "correct" the user and fallback on gcc
when that variable is wrong. Fail and point at the problem instead. No
one ever pays attention to build logs; as shown by new warnings being
submitted on a regular basis.

This also removes the local variable "XCC", the name of which was
collision-prone and confusingly capitalized.

Signed-off-by: Marc Herbert <marc.herbert@intel.com>

This is required to allow building XTOS with newer, clang-based, Cadence
toolchains as just submitted for ACP_7_0 in thesofproject#9351

Signed-off-by: Marc Herbert <marc.herbert@intel.com>

It was awkward to depend on ZEPHYR_TOOLCHAIN_VARIANT when the testbench
has absolutely nothing to do with it.

Signed-off-by: Marc Herbert <marc.herbert@intel.com>