cmd/link: don't mangle string symbol names

cmd/link: don't mangle string symbol names

String symbol names could contain weird characters as we put the
string literal into the symbol name. So it may appear to need
mangling. However, as string symbols are grouped into a single
"go:string.*" symbol, the individual symbol names actually don't
matter. So don't mangle them.

Also make the mangling code more defensive in case of weird
symbol names.

Fixes #62098.

Change-Id: I533012567a9fffab69debda934f426421c7abb04
Reviewed-on: https://go-review.googlesource.com/c/go/+/520856
Reviewed-by: Than McIntosh <thanm@google.com>
Run-TryBot: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>