Add support for the RISC-V architecture

[aaronfranke]

Implements godotengine/godot-proposals#3374 on master. Only Clang Debug builds work on RISC-V right now.

[AaronRecord]

Why isn't this

Suggested change

	supported_arch = env["bits"] == "64" and env["arch"] != "arm64" and not env["arch"].startswith("rv")
	supported_arch = env["bits"] == "64" and env["arch"] != "arm64" and env["arch"] != "rv64"

?

[aaronfranke]

In the future we might want to allow specifying a custom -march string, like "rv64gcv". There's not a lot of use for this at the moment so I didn't bother supporting it in the SConstruct file, but the startswith check is a simple improvement over checking string inequality that will work for that case. Plus it's possible that someone would want to make a 32-bit or 128-bit version, and startswith also works for that case with no downsides.

[akien-mga]

Nitpick, but since we import platform already, I'd prefer simply using platform.machine() instead of re-importing it.

[aaronfranke]

Good catch, this was just a blind copy from SConstruct so I missed that we already import platform.

[akien-mga]

Thanks!