Fix parsing of target architecture flags with values that include arm64

[michelleangela]

This change addresses item #1735

• The value arm64 used for target architecture flags would get parsed as arm which resulted in sending the incorrect IntelliSenseMode to the vscode-cpptools extension.
• If gcc compiler is used, it would return clang compiler for IntelliSenseMode instead of gcc.

This changes

• Code that parses value of target architecture flags. If value includes arm64, then it will return arm64 architecture instead of arm to use as the architecture variant of the IntelliSenseMode.
• Code that returns IntelliSenseMode based on compiler path input will return gcc-<arch> IntelliSenseMode if gcc compiler is used.

The purpose of this change

To return the correct compiler-architecture variant for IntelliSenseMode to vscode-cpptools extension.

Other Notes/Information