[lld] [LLD][COFF] Move symbol mangling and lookup helpers to SymbolTable class (NFC) (PR #122836)

[Martin Storsjö via llvm-commits]

https://github.com/mstorsjo approved this pull request.

LGTM - I didn't check the contents of the moved functions, but I presume it's only unmodified copies, modulo different references for where to find values like `ctx.config.machine` vs `machine` etc.

I meant to ask what the need for this is, as we usually only need to treat i386 differently, but as there are more patches that require it, I guess I'll see in a moment :-)

https://github.com/llvm/llvm-project/pull/122836