make GCC version detection OS-independent

make GCC version detection OS-independent

Previous design put GCC version detection inside of OSNAME 'WINNT'.
However, such detections are required for 'Linux' and possibly other
OS'es as well. For example, there is usage of the GCC versions
in Makefile.arm64. When compiling on Linux machine, in the previous
design, Markfile.arm64 will not know the correct GCC version.

The fix is to move GCC version detection into common part, not
wrapped by anything.

Signed-off-by: Guodong Xu <guodong.xu@linaro.com>