[WebAssembly] The .reloc directive is not implemented

[SingleAccretion]

Reproduction:

; reloc.S
  .data
  .reloc ., R_WASM_MEMORY_ADDR_I32, some_symbol

> clang -c reloc.S
> error: unknown relocation name

This is due to the missing implementation of getFixupKind (and possibly some other pieces).

[llvmbot]

@llvm/issue-subscribers-backend-webassembly

Author: None (SingleAccretion)

Reproduction: ```asm ; reloc.S .data .reloc ., R_WASM_MEMORY_ADDR_I32, some_symbol ``` ``` > clang -c reloc.S > error: unknown relocation name ``` This is due to the missing implementation of [`getFixupKind`](https://github.com/llvm/llvm-project/blob/67ad23fe17a516b4b66d8137f77ab0ba3b6fc95b/llvm/lib/MC/MCObjectStreamer.cpp#L694-L697) (and possibly some other pieces).