cmd/go: recognize DLL magic from llvm binaries

When using LLD with c-shared, the magic in the output DLL is slightly
different than for EXEs.

Change-Id: Icc5f34f7bb61f11a9d75494236b7797cc1988b40
Reviewed-on: https://go-review.googlesource.com/c/go/+/291638


Trust: Jason A. Donenfeld <Jason@zx2c4.com>
Run-TryBot: Jason A. Donenfeld <Jason@zx2c4.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>