Add a rudimentary wasm64 module with intrinsics

[alexcrichton]

This follows the x86/x86_64 pattern where wasm64 simply reexports the
wasm32 module for now. The documentation is pretty lacking since this
is just the start as well.

[rust-highfive]

r? @Amanieu

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[Amanieu]

The memory intrinsics currently use the i32 versions of the LLVM intrinsics (llvm.wasm.memory.grow.i32 & llvm.wasm.memory.size.i32). Should this be changed to use 64-bit variants on wasm64?

[devsnek]

I think you need to update some of the intrinsics, here's the changed file from my pr: https://github.com/rust-lang/stdarch/blob/77af60f13156471dc39c806b4be124ef591dc37b/crates/core_arch/src/wasm/memory.rs

[alexcrichton]

Sure, I can update the memory intrinsics as well. FWIW my impression is that the LLVM backend here is still pretty immature, I've gotten crashes in LLVM not compiling in release mode and it also can't compile this repository's tests just yet due to odd relocation errors. I highly doubt this works "as-is" but inching more towards wasm64 I figure is good.