Improve the STACKPROTECTOR check

The Linux kernel 4.16-4.17 has a weird STACKPROTECTOR configuration:
CC_STACKPROTECTOR_NONE -- stackprotector is disabled;
CC_STACKPROTECTOR_REGULAR -- similar to current STACKPROTECTOR;
CC_STACKPROTECTOR_STRONG -- similar to current STACKPROTECTOR_STRONG;
CC_STACKPROTECTOR_AUTO -- the best stack-protector that compiler provides.
These options are mutually exclusive.

Let's improve the STACKPROTECTOR check:
- Add CC_STACKPROTECTOR_REGULAR as a valid alternative name of this option;
- Add CC_STACKPROTECTOR_STRONG to avoid false negative result;
- Add CC_STACKPROTECTOR_AUTO hoping that it enables at least STACKPROTECTOR.

The STACKPROTECTOR_STRONG check still requires explicit configuration, not
CC_STACKPROTECTOR_AUTO.

Thanks to @izh1979 for the idea