Exports correct tooling in gmake(2) if GCC is specified

[nickclark2016]

What does this PR do?

When using gmake or gmake2 exporter and GCC toolchain, the CC/CXX/AR variables used the environment default rather than being set to GCC tooling. This forces the Makefile to use gcc, g++, and ar.

How does this PR change Premake's behavior?

See above

Anything else we should know?

Closes #266 #1860

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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