It turns out -iex argument is not supported by all gdb versions,
but as we need to add the auto-load safe path before loading the
inferior, test -iex support first and skip the test if it's not
available.

We should still update our builders though.

Change-Id: I355697de51baf12162ba6cb82f389dad93f93dc5
Reviewed-on: https://go-review.googlesource.com/4070Reviewed-by: Ian Lance Taylor <iant@golang.org>