[Hexagon] Alter meaning of versionless -mhvx

The documentation for the official (downstream) Qualcomm Hexagon Clang
states that -mhvx sets the HVX version to be the same as the CPU version.
The current implementation upstream would use the most recent versioned
-mhvx= flag first (if present), then the CPU version. Change the upstream
behavior to match the documented behavior of the downstream compiler.