* use the real start code for LLVM backend with x86_64-linux
- there is still a check for zig_backend after initializing the TLS
area to skip some stuff.
* introduce new AIR instructions and implement them for the LLVM
backend. They are the same as `call` except with a modifier.
- call_always_tail
- call_never_tail
- call_never_inline
* LLVM backend calls hasRuntimeBitsIgnoringComptime in more places to
avoid unnecessarily depending on comptimeOnly being resolved for some
types.
* LLVM backend: remove duplicate code for setting linkage and value
name. The canonical place for this is in `updateDeclExports`.
* LLVM backend: do some assembly template massaging to make `%%`
rendered as `%`. More hacks will be needed to make inline assembly
catch up with stage1.
To unify the wasm backend with the other backends, we will now call `generateSymbol` to
lower a Decl into bytes. This means we also have to change some function signatures
to comply with the linker interface.
Since the general purpose generateSymbol is less featureful than wasm's, some tests are
temporarily disabled.
This makes all union test cases succeed.
`rem` was also implemented as all we had to do is enable the instruction.
Loading and storing values based on ABI-size was simplified to a direct abiSize() call.
We also enabled all the newly passing test cases and disable them for all non-passing backends.
All of those test cases were verified to see if they perhaps already pass for the c-backend.
