Target specs v3

[Firestar99]

Wanted to simply target specs even more:

• remove crate rustc_codegen_spirv-target-specs
  • this removes all the spirv-unknown-*.json target spec files in the repo
• generate target specs on the fly in spirv-builder
• spirv-builder no longer checks that target env is a known valid one:
  

  rust-gpu/crates/rustc_codegen_spirv-types/src/target.rs

  Lines 8 to 11 in 823b0c2

  	/// The constructors [`Self::from_target`] or [`Self::from_env`] only check whether the target is well-formed, not
  	/// whether it is valid. Since `spirv-builder` is backwards compatible with older rust-gpu compilers, only the compiler
  	/// itself knows what targets it can and cannot support. This also allows adding new targets to the compiler without
  	/// having to update `spirv-builder` and `cargo-gpu`.

• target validation within the compiler is untouched, to be done in a followup PR
  • Learning from enum SpirvTargetEnv containing all available targets #311 having had a lot of discussions on how to represent targets properly
• cargo-gpu see Target specs v3 cargo-gpu#128
  • this cargo-gpu action shows this PR being tested against various rust-gpu releases and old revs
  • rust-gpu versions after [this PR] will still work with older cargo-gpu versions, until we update target specs in rust-gpu again (not planned anytime soon)
• in total with cargo-gpu, ~600 lines have been removed (including json)