6.x - Initial GCC toolset implementation

[nickclark2016]

What does this PR do?

Adds a toolset abstraction and implementation for GCC.

How does this PR change Premake's behavior?

No outwards facing behavioral changes.

Anything else we should know?

Each toolset will need to be registered in order to be used by the "toolset" API.

Did you check all the boxes?

• [x] Focus on a single fix or feature; remove any unrelated formatting or code changes
• [x] Add unit tests showing fix or feature works; all tests pass
• [x] Mention any related issues (put closes #XXXX in comment to auto-close issue when PR is merged)
• [x] Follow our coding conventions
• [x] Minimize the number of commits
• [x] Align documentation to your changes

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