cmd/link: report error if builtin referenced but not defined

When the compiler refers to a runtime builtin, it emits an indexed
symbol reference in the object file via predetermined/preassigned ID
within the PkgIdxBuiltin pseudo-package. At link time when the loader
encounters these references, it redirects them to the corresponding
defined symbol in the runtime package. This redirection process
currently assumes that if a runtime builtin is referenced, we'll
always have a definition for it. This assumption holds in most cases,
however for the builtins "runtime.racefuncenter" and
"runtime.racefuncexit", we'll only see definitions if the runtime
package we're linking against was built with "-race".

In the bug in question, build passes "-gcflags=-race" during
compilation of the main package, but doesn't pass "-race" directly to
'go build', and as a result the final link combines a
race-instrumented main with a non-race runtime; this results in R_CALL
relocations with zero-valued target symbols, resulting in a panic
during stack checking.

This patch changes the loader's resolve method to detect situations
where we're asking for builtin "runtime.X", but the runtime package
read in doesn't contain a definition for X.

Fixes #42396.

Change-Id: Iafd38bd3b0f7f462868d120ccd4d7d1b88b27436
Reviewed-on: https://go-review.googlesource.com/c/go/+/267881


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