cmd/link: detect glibc vs musl ldso at link time

cmd/link: detect glibc vs musl ldso at link time

Doing the test at link time lets us distribute one Linux toolchain
that works on both glibc-based and musl-based Linux systems.
The old way built a toolchain that only ran on one or the other.

Fixes #54197.

Change-Id: Iaae8c274c78e1091eee828a720b49646be9bfffe
Reviewed-on: https://go-review.googlesource.com/c/go/+/420774
Auto-Submit: Russ Cox <rsc@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>