add basic support for rgbds assembly

[terinjokes]

This adds basic support for assembling and linking Game Boy and Game Boy Color games with RGBDS's rgbasm and rgblink. This also adds knowledge of the Sharp SM83 CPU family, the 8080/Z80-like CPU used on the device's SoC.

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An example of this being used can bee seen in the two included tests, as we'll as a fork of pokered.

As demonstrated in test case "2 rgbfix" as well as the pokered fork, many projects will need to call rgbfix to setup the ROM headers to the expected (and computed, in the case of checksum) values on top of the output of executable(). Unfortunately, it's not possible to have the linker automatically call rgbfix, and I wasn't able to find a good place in Meson to add that call. I'm open to ideas as possible alternatives to the custom_target I've documented.

[terinjokes]

I'm open to ideas as possible alternatives to the custom_target I've documented.

I've implemented one possible alternative by adding an rgbds module. However, for the time being I've pushed that work to a separate branch, https://github.com/terinjokes/meson/tree/rgbds-module

[dcbaker]

A module is probably the most sensible way to go about replacing the custom target.

The code here all looks good to me. The one concern I have is that we now have two different languages implementing .asm support. While that should be safe across subproject boundaries, if someone has both the rgbds and asm language enabled at the same time, I suspect that we'll have issues. We probably need at the very least a warning, if not an error, if both get enabled at the same time.

[terinjokes]

In this specific example, you do already get an error if you declare both in project() as both nasm and rgbasm have non-intersecting CPU constraints. (I was able to workaround with add_languages to be able to set nasm as native)

meson.build:1:0: ERROR: ASM compiler 'nasm' does not support sm83 CPU family

Looking past this specific case (eg, if this PR was adding fasm instead), I can imagine the issues. I'll look at where I could add a warning or error.

[dcbaker]

hmmm, if they're for different machines then that's not actually a problem. As long as the rgdbs only supports the sm83 architecture I think that's fine. You could, In theory, be building an emulator for the build machine and a rom image for a host machine (ie, for the emulator) as a single invocation, and the emulator using nasm to target x86 and the rom using rgbds would be a valid configuration