misc/ios: evaluate symlinks before comparing GOROOT and GOPATH
misc/ios: evaluate symlinks before comparing GOROOT and GOPATH

CL 163726 added workarounds to keep the iOS builders happy in
a symlinked temporary dir.

The workarounds also made the tests more realistic and improved
performance. Keep them but also handle symlinks better in the
exec wrapper.

Change-Id: Iaa2c03a1a3fb3aa5aaf62d79d52b63d5d8f11db5
Reviewed-on: https://go-review.googlesource.com/c/164698
Reviewed-by: Bryan C. Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>

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