[WebAssembly][MC] Simplify WasmObjectWriter::recordRelocation. NFC.

WebAssembly doesn't support PC relative relocation or relocation
expressions that can't be reduced to single symbol.

The only support for we have for fixups involving two symbols are when
both symbols are defined and withing the same section.  In this case
evaluateFixup will already have evaluated to the expression before
calling recordRelocation.

llvm-svn: 369317