cmd/link: generate an error if the entry symbol cannot be found

cmd/link: generate an error if the entry symbol cannot be found

If the entry symbol cannot be found (for example, a new port is being brought
up and no rt0 code has been provided), the linker will currently panic. Rather
than panicing, generate an error that aids in debugging:

	missing entry symbol "_rt0_arm64_openbsd"

Change-Id: I9cc38eaab48f730d596ca7fa9e9e3d68250ae4d3
Reviewed-on: https://go-review.googlesource.com/c/go/+/430555
Auto-Submit: Jenny Rakoczy <jenny@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Jenny Rakoczy <jenny@golang.org>
Run-TryBot: Cherry Mui <cherryyz@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>