cmd/internal/obj/x86: still use (fake) local exec TLS mode on android/386

golang.org/cl/16383 broke android/386 because by a sort of confluence of hacks
no TLS relocations were emitted at all when Flag_shared != 0. The hack in
runtime/cgo works as well in a PIE executable as it does with a position
dependent one, so the simplest fix is to still emit a R_TLS_LE reloc when goos
== "android".

A real fix is to use something more like the IE model code but loading the
offset from %gs to the thread local storage from a global variable rather than
from a location chosen by the system linker (this is how android/arm works).

Issue golang#9327.

Change-Id: I9fbfc890ec7fe191f80a595b6cf8e2a1fcbe3034

Author: mwhudson

src/cmd/internal/obj/x86/asm6.go

@@ -2536,7 +2536,7 @@ func vaddr(ctxt *obj.Link, p *obj.Prog, a *obj.Addr, r *obj.Reloc) int64 {
 			log.Fatalf("reloc")
 		}
 
-		if ctxt.Flag_shared == 0 {
+		if ctxt.Flag_shared == 0 || isAndroid {
 			r.Type = obj.R_TLS_LE
 			r.Siz = 4
 			r.Off = -1 // caller must fill in