Added support for building for tvOS

This change is just to add support for building for tvOS.
It is exactly the same as iOS, just using a different SDK.

I had to change the two lines for libjpeg-turbo so that
it will run for both tvOS and iOS.

BUG=skia:

Change-Id: I6ae5fc4257df74c0f321e5d2d71584f6a52ec3a6
Reviewed-on: https://skia-review.googlesource.com/9660
Commit-Queue: Mike Klein <mtklein@chromium.org>
Reviewed-by: Mike Klein <mtklein@chromium.org>
diff --git a/gn/BUILDCONFIG.gn b/gn/BUILDCONFIG.gn
index 6ec5e67..2b3ae33 100644
--- a/gn/BUILDCONFIG.gn
+++ b/gn/BUILDCONFIG.gn
@@ -37,7 +37,8 @@
 
 is_android = current_os == "android"
 is_fuchsia = current_os == "fuchsia"
-is_ios = current_os == "ios"
+is_ios = current_os == "ios" || current_os == "tvos"
+is_tvos = current_os == "tvos"
 is_linux = current_os == "linux"
 is_mac = current_os == "mac"
 is_win = current_os == "win"