Rework the build system, support more systems

[DCNick3]

This PR (once again) reworks the way asm for different platforms is built, as well as adding more CI targets to test it.

For the environment detection I decided to use pattern matching as much as possible, as it is easier to reason about. I have also made the code explicitly bail from building asm if the platform does not seem to be supported.

As for the logic changes, I had to change this stuff:

• do the symbol underscode prefixing only on x86 windows and x86/x86_64 macOS (instead of any x86_64 unix-like os)
• do not enable stack protectors when building for windows with a MinGW toolchain. This adds a dependency on stack checking library that is not linked by rust and I could not make it link correctly otherwise (tried stuff from https://github.com/mgeier/libflac-sys/pull/29). Stack protectors are disabled on windows upstream too (see https://github.com/rdp/ffmpeg-windows-build-helpers/issues/380#issuecomment-475278667)

For the CI I changed up the matrix a bit:

• build for more rust targets, as well as combinations of "with nasm" and "without nasm"
• do not build with nightly rust. Otherwise there were just too much jobs. I believe there is not much value in testing with nightly if unstable features are not used. Breaking stable user code is not something rust does that often

Currently, all targets pass CI: https://github.com/DCNick3/openh264-rust/actions/runs/4168027112/jobs/7214327810

It would be nice to have tests covering arm platforms, and maybe even more, but github hosted runners are x86-only. There are some ways around it (like using qemu), but it didn't seem trivial, so I didn't do it.

Could use at least "it links" check though :thinking:

[DCNick3]

Well, whoops. Arm is not something supported by nasm, upstream uses a different assembler for it. Ig I will disable the asm support on arm for now then

[DCNick3]

Added a cargo config for linux arm targets to use a correct linker, for some reason it was using an x86 one

Now the CI also tries to link the arm targets (even though the asm is not used there yet)

https://github.com/DCNick3/openh264-rust/actions/runs/4168902957/jobs/7216231574

[ralfbiedert]

Thank you so much, this is great!