runtime: don't use thread local storage before it is set up on iOS
runtime: don't use thread local storage before it is set up on iOS

CL 138675 added a call to runtime.save_g which uses thread local
storage to store g. On iOS however, that storage was not initialized
 yet. Move the call to below _cgo_init where it is set up.

Change-Id: I14538d3e7d56ff35a6fa02c47bca306d24c38010
Reviewed-on: https://go-review.googlesource.com/c/150157
Run-TryBot: Elias Naur <993af7099cb24ce1a88d64108ae1e8b0385f2fc0@gmail.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Cherry Zhang <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>

cloudflare/go,josharian/go,golang/go,CAFxX/go,nawawi/go,cloudflare/go,cloudflare/go,akutz/go,golang/go,nawawi/go,akutz/go,golang/go,CAFxX/go,CAFxX/go,cloudflare/go,akutz/go,akutz/go,nawawi/go,akutz/go,golang/go,cloudflare/go,CAFxX/go,golang/go,CAFxX/go,josharian/go,nawawi/go,josharian/go,CAFxX/go,akutz/go,nawawi/go,golang/go,josharian/go,nawawi/go,CAFxX/go,josharian/go,nawawi/go,cloudflare/go,akutz/go,josharian/go,golang/go,CAFxX/go,akutz/go,nawawi/go,cloudflare/go,josharian/go,josharian/go,cloudflare/go,golang/go