Reduced necessary compiler options

For a properly-configured toolchain, many of the previously-necessary compiler options can be omitted. Update the build system to use these defaults.

The expected toolchain targets `sh4aeb-nofpu-elf` and knows where libfxcg's `include` and `lib` directories are. The binary prefix is `prizm-`, so `gcc` is `prizm-gcc`. A template for building such a toolchain may be found in the PrizmSDK [dockerfile].

For users using a different compiler, it should be feasible to provide `prizm-gcc` and friends as shell scripts wrapping the native compilers (providing the options for necessary include and library paths, perhaps `-m4a-nofpu -mb` for the old `sh3eb-elf` toolchain, etc), but this has not been tested.

[dockerfile]: https://bitbucket.org/tari/docker-prizmsdk