[release-branch.go1.16] cmd/link: disable plugin support if cgo is disabled

Functional plugin support requires cgo to be enabled. Disable
it if the environment has disabled cgo.

This prevents unexpected linker failures when linking large
binaries with cgo disabled which use the plugin package.

Fixes #45832

Change-Id: Ib71f0e089f7373b7b3e3cd53da3612291e7bc473
Reviewed-on: https://go-review.googlesource.com/c/go/+/314449
Run-TryBot: Paul Murphy <[email protected]>
Reviewed-by: Cherry Zhang <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Trust: Lynn Boger <[email protected]>
(cherry picked from commit 983dea9)
Reviewed-on: https://go-review.googlesource.com/c/go/+/316329
Reviewed-by: Cherry Mui <[email protected]>
Trust: Dmitri Shuralyov <[email protected]>

Author: pmur

src/cmd/link/internal/ld/lib.go

@@ -533,7 +533,10 @@ func (ctxt *Link) loadlib() {
 	// up symbol by name may not get expected result.
 
 	iscgo = ctxt.LibraryByPkg["runtime/cgo"] != nil
-	ctxt.canUsePlugins = ctxt.LibraryByPkg["plugin"] != nil
+
+	// Plugins a require cgo support to function. Similarly, plugins may require additional
+	// internal linker support on some platforms which may not be implemented.
+	ctxt.canUsePlugins = ctxt.LibraryByPkg["plugin"] != nil && iscgo
 
 	// We now have enough information to determine the link mode.
 	determineLinkMode(ctxt)