Improve README/help regarding build types

[edubart]

A suggestion is to add those options to the make help target. Something like:

help:
	@echo 'Main targets:'
	@echo '* all - Build the src/ and uarch/ code. To build from a clean clone, run: make submodules all'
	@echo '        Additionally, to specify the build type, pass yes to one of:'
	@echo '        - sanitize, debug, relwithdebinfo, *release*, coverage. e.g: make sanitize=yes

Another point, a bit unrelated to this PR is removing the Emscripten documentation from the README.md. A brief comment pointing to the wiki should suffice. Should we do the same for the mingw toolchain?

Originally posted by @mpolitzer in #250 (comment)