Switch platform-NDK unwinder to LLVM libunwind.a prebuilt

Link against the libunwind.a shipped with the NDK so it matches what's
linked into the NDK's libc++_shared.so.

Bug: http://b/153025717
Test: device boots, manually inspect some linker command lines
Change-Id: Icc79844f5e70f0eaa97ea758449c30fbddd030d2

cc/stl.go

@@ -188,12 +188,7 @@ func (stl *stl) deps(ctx BaseModuleContext, deps Deps) Deps {
 		if needsLibAndroidSupport(ctx) {
 			deps.StaticLibs = append(deps.StaticLibs, "ndk_libandroid_support")
 		}
-		// TODO: Switch the NDK over to the LLVM unwinder for non-arm32 architectures.
-		if ctx.Arch().ArchType == android.Arm {
-			deps.StaticLibs = append(deps.StaticLibs, "ndk_libunwind")
-		} else {
-			deps.StaticLibs = append(deps.StaticLibs, "libgcc_stripped")
-		}
+		deps.StaticLibs = append(deps.StaticLibs, "ndk_libunwind")
 	default:
 		panic(fmt.Errorf("Unknown stl: %q", stl.Properties.SelectedStl))
 	}