Add support for optimizing GCC command line flags

[ChrisCummins]

This issue tracks progress on merging @hughleat's new GCC environment into CompilerGym.

Progress

• Merge initial implementation (Gcc Port for CompilerGym #314).
• Add a "GCC Environment Reference" document to the docs.
• Complete the new action space schema (Extend action spaces to be composable and continuous. #369).
• Extend the microbenchmark suite to GCC.
• Port GCC action space to the new schema to expose a Tuple / Dict space with all options set by a single action.
• Complete the new multi-file Benchmark proto schema (Extend RPC interface to support multi-file benchmarks #325).
• Use multi-file Benchmarks to enable the GCC service to accept as a benchmark a set of files, not a single pre-processed source.