gdbserver: When attaching, add process before lwps

gdbserver: When attaching, add process before lwps

The recent BP/WP changes for AArch64 swapping the order in add_lwp()
so that the process was added before the lwp. This was due to the lwp
creation requiring the process data.

This also needs changing in linux_attach().

Also add additional checks to make sure cannot attach to the same
process twice. Add test case for this - do this by splitting
attach.exp into distinct pass and error case sections.

Fixes gdb.server/ext-attach.exp on Aarch64.

gdb/gdbserver/ChangeLog:

	* linux-low.c (linux_attach): Add process before lwp.
	* server.c (attach_inferior): Check if already attached.

gdb/testsuite/ChangeLog:

	* gdb.base/attach.exp: Add double attach test.