tools/thermal: tmon: support cross-compiling

We might want to prepare CFLAGS outside of this Makefile, so don't
overwrite its initial value.

Then, support $(CROSS_COMPILE), so we can use a cross-compile toolchain.

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Acked-by: Jacob Pan <jacob.jun.pan@linux.intel.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>