cmd/link: always disable lazy PLT resolution when dynamically linking Go

Go cannot allow lazy PLT resolution when calling between Go functions because
the lazy resolution can use more stack than is available. Lazy resolution is
disabled by passing -z now to the system linker, but unfortunately was only
passed when linking to a Go shared library. That sounds fine, but the shared
library containing the runtime is not linked to any other Go shared library but
calls main.init and main.main via a PLT, and before this fix this did use lazy
resolution. (For some reason this never caused a problem on intel, but it
breaks on ppc64le). Fortunately the fix is very simple: always pass -z now to
the system linker when dynamically linking Go.

Change-Id: I7806d40aac80dcd1e56b95864d1cfeb1c42614e2
Reviewed-on: https://go-review.googlesource.com/15870Reviewed-by: Ian Lance Taylor <iant@golang.org>

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

@@ -1006,7 +1006,7 @@ func hostlink() {
 		argv = append(argv, "-shared")
 	}
 
-	if Linkshared && Iself {
+	if Iself && DynlinkingGo() {
 		// We force all symbol resolution to be done at program startup
 		// because lazy PLT resolution can use large amounts of stack at
 		// times we cannot allow it to do so.