Introduce sound related system calls

[ChinYikMing]

mus2midi utility is referenced by Mus2Midi .

Tested in Ubuntu 20.04.

Close #152

[jserv]

Update docs/syscall.md and provide a test program in tests directory.

[ChinYikMing]

Provide a test program in tests directory.

Sound version of doom.elf is come with the commit, simply run make doom in sound branch

[jserv]

Sound version of doom.elf is come with the commit, simply run make doom in sound branch

No, let's minimize the necessary changes. You should provide a minimal C program that demonstrates the use of the proposed system calls instead of making extensive modifications to Doom. Once this pull request is merged, we can proceed with the Doom and Quake pull requests in their respective repositories.

[jserv]

Simple SDL2 Audio operates independently of SDL2 Mixer and is designed to play a single music file at a time. It also supports an unlimited number of sounds, achieving audio mixing natively without the need for an external mixer. It might be useful.

[ChinYikMing]

Update docs/syscall.md and provide a test program in tests directory.

You may test the result of integration using the test program in tests/sound directory, use =h option to check the usage.

To test all at once, simply run ../../build/rv32emu sound.elf =m d_e1m1.mus =s dspistol =upmv =srep in tests/sound directory.

[ChinYikMing]

Move the content of src/mus2midi.c to src/syscall_sdl.c since the former is short and dedicated to the latter.

tests/sound/sound.c requires src/mus2midi.c, I think we should leave it to be.

I made a mistake which mus2midi is only used in emulator since it control the sound, I will adopt the required change.

[jserv]

Improve git commit messages.

[jserv]

Thank @ChinYikMing for contributing!