Disable exec-dependent follow vfork tests for remote

Disable exec-dependent follow vfork tests for remote

The native-extended-gdbserver target now supports fork events and
follow fork, but it does not yet support exec events.  Some of the
tests in gdb.base/foll-vfork.exp depend on exec events.  This patch
disables those tests for remote targets.  We can re-enable these
once the exec event support goes in.

gdb/testsuite/

	* gdb.base/foll-vfork.exp (main): Disable exec-dependent
	tests for remote targets by checking is_target_gdbserver.