This patch adds to rebuild-testbench option -x <platform> that
can be used to build testbench for xt-run execution. The enhanced
script reuses native testbench build but with CC, LD, LDFLAGS,
etc. defines to use the xt-xcc compiler for build.
Currently TGL (HiFi3) is the only supported platform. More will
be added later.
Signed-off-by: Seppo Ingalsuo <seppo.ingalsuo@linux.intel.com>