compiler: gcc: add support for tuning mcpu option for ARC targets

ARC processors are highly configurable, so ARC toolchain supports
big amount of mcpu options for all that HW configurations.
As difference in many configurations among the same processor
family usually doesn't affect Zephyr code we don't want
to create Kconfig option for each possible CPU configuration
(just to map Kconfig option to correspondent mcpu value
in 'cmake/gcc-m-cpu.cmake').
Instead we prefer being able to set proper mcpu value
individually for any ARC SoC and using 'cmake/gcc-m-cpu.cmake'
just for reasonable defaults.

To apply SoC-related changes for build-options on early stages of building
'tune_build_ops.cmake' should be created in appropriate SoC directory.
Example:
 ./soc/arc/snps_qemu/tune_build_ops.cmake
File content:
 set(GCC_ARC_TUNED_CPU hs4xd)

Signed-off-by: Nikolay Agishev <agishev@synopsys.com>