Rename gmake to gmakelegacy

[nickclark2016]

What does this PR do?

Renames the gmake exporter to gmakelegacy. Provides an alias from gmake2 to gmake. Follow up PR will handle renaming gmake2 to gmake and deprecating the gmake2 exporter name in favor of gmake.

How does this PR change Premake's behavior?

This will break any existing scripts utilizing gmake currently that do not work with gmake2 (should be a minimal group of users). Users can opt into the old exporter by using gmakelegacy instead.

Anything else we should know?

The name gmakelegacy was derived from both a desire to keep scripts using action:gmake* filters working, while explicitly denoting that the exporter is deprecated.

Did you check all the boxes?

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

You can now support Premake on our OpenCollective. Your contributions help us spend more time responding to requests like these!