standalone: use GCC_VERSION defined in compiler-gcc.h

Now GCC_VERSION is defined in include/linux/compiler-gcc.h
(with a little different definition).
Use it and delete the one in examples/standlone/stub.c.

This should work on Clang too because __GNUC__, __GNUC_MINOR__,
__GNUC_PATCHLEVEL__ are also defined on Clang.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Cc: Jeroen Hofstee <jeroen@myspectrum.nl>