Cross compilation compatibility builds

[Adoni5]

Main changes

• After hitting FEATURE: Conditional compilation for aarch64, arm_neon, and simde #13 (comment) a couple of time whilst trying to compile mappy-rs wheels for multiple architectures, I realised that checking target_arch in the build.rs script of minimap2-sys, only considers the host architecture and not the target triple.
  This meant that the target_specific function

  minimap2-rs/minimap2-sys/build.rs

  Lines 153 to 154 in bb118a5

  	#[cfg(target_arch = "x86_64")]
  	fn target_specific(cc: &mut cc::Build) {

  was being run for x86_64 every time, irrespective of which cross compilation was occurring.
  To catch this I have changed the target_specific function and dropped conditional compilation for it based on target architecture, and instead used a check against the TARGET env variable inside the function, adding flags based on whether the target triple is x86 or aarch64.

• In the minimap2-rs library code, I had to add a From trait to convert presets to C strings for both i8 and u8 types. I also conditionally compile any sections where pointers are passed through to the underlying FFI functions based on architecture. In linux aarch64/armv7 cross compilation, the underlying C compiler expects u8 pointers, whereas linux x86_64, macOS x86_64, and macOS aarch64 expect i8 pointers. I have not been able to test if this breaks native compilation on linux aarch64.

Other changes

1. For linux aarch64, for some reason to cross compile, the C99 standard had to be enabled, due to for loop constructions in minimap2. I've added this to the simde feature function, https://github.com/Adoni5/minimap2-rs/blob/d804692362e414058123de21d3464f6e7d615f52/minimap2-sys/build.rs#L167
   But I don't know if that's the best place to put it?

2. I added fakeminimap2 and minimap2-sys as cargo workspaces in the root Cargo.toml, simply so that rust-analyzer would work in the directory. This can be removed if you prefer, but I don't think it has any knock on effects otherwise.

3. Built fakeminimap2 with local minimap2-rs crate - no reason, just seems semi-logical as it seems useful for testing.

[Adoni5]

Final note - the Cargo.toml for minimap2-rs currently points to the local minimap2-sys. This should be updated when the new version of minimap2-sys is up on crates.io

[jguhlin]

Amazin!. I'm recovering from a conference but will get this pushed out in the next few days.