cmd/link: only exclude C-only symbols on darwin

C-only symbols are excluded from pclntab because of a quirk of darwin,
where functions are referred to by an exported symbol so dynamic
relocations de-duplicate to the host binary module and break unwinding.

This doesn't happen on ELF systems because the linker always refers to
unexported module-local symbols, so we don't need this condition.
And the current logic for excluding some functions breaks the module
verification code in moduledataverify1. So disable this for plugins
on linux.

(In 1.9, it will probably be necessary to introduce a module-local
symbol reference system on darwin to fix a different bug, so all of
this onlycsymbol code made be short-lived.)

With this CL, the tests in CL 35116 pass.

Change-Id: I517d7ca4427241fa0a91276c462827efb9383be9
Reviewed-on: https://go-review.googlesource.com/35190Reviewed-by: Michael Hudson-Doyle <michael.hudson@canonical.com>
Run-TryBot: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

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

@@ -168,7 +168,7 @@ func container(s *Symbol) int {
 	if s == nil {
 		return 0
 	}
-	if Buildmode == BuildmodePlugin && onlycsymbol(s) {
+	if Buildmode == BuildmodePlugin && Headtype == obj.Hdarwin && onlycsymbol(s) {
 		return 1
 	}
 	// We want to generate func table entries only for the "lowest level" symbols,