Invalid _mulx_u32 CMake check

[fweimer-rh]

This build-time check is invalid:

https://github.com/awslabs/aws-c-common/blob/7959065399a86be95073085be381eedfcbe93538/cmake/AwsFeatureTests.cmake#L36..L43

• GCC defines _mulx_u32 in a header file so the test always fails.
• A build-time test for _mulx_u32 does not tell us anything about run-time availability.

Found as part of:

• https://fedoraproject.org/wiki/Changes/PortingToModernC
• https://fedoraproject.org/wiki/Toolchain/PortingToModernC

[TingDaoK]

The issue has been resolved by #974