cmd/link: exit before Asmb2 if error

If there are already errors emitted, don't run the Asmb2 pass
and just exit. At the point of Asmb2 relocations are already
resolved and errors should have been reported, if any. Asmb2 is
unlikely to emit additional useful users errors. Instead, the
invalid input may cause inconsistencies and crash the linker, or
it may emit some internal errors which are more confusing than
helpful. Exit on error before Asmb2.

Fixes #43748.

Change-Id: Icf6e27f2eef5b6259e921ec0e64bebad5dd805f8
Reviewed-on: https://go-review.googlesource.com/c/go/+/284576


Trust: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>