Fix "Remote 'g' packet reply is too long" problems with multiple inferiors

Fix "Remote 'g' packet reply is too long" problems with multiple inferiors

When debugging two inferiors (or more) against gdbserver, and the
inferiors have different architectures, such as e.g., on x86_64
GNU/Linux and one inferior is 64-bit while the other is 32-bit, then
GDB can get confused with the different architectures in a couple
spots.

In both cases I ran into, GDB incorrectly ended up using the
architecture of whatever happens to be the selected inferior instead
of the architecture of some other given inferior:

#1 - When parsing the expedited registers in stop replies.

#2 - In the default implementation of the target_thread_architecture
     target method.

These resulted in instances of the infamous "Remote 'g' packet reply
is too long" error.  For example, with the test added in this commit,
we get:

~~~
  Continuing.
  Remote 'g' packet reply is too long (expected 440 bytes, got 816 bytes): ad064000000000000[snip]
  (gdb) FAIL: gdb.multi/multi-arch.exp: inf1 event with inf2 selected: continue to hello_loop

  c
  Continuing.
  Truncated register 50 in remote 'g' packet
  (gdb) PASS: gdb.multi/multi-arch.exp: inf2 event with inf1 selected: c
~~~

This commit fixes that.

gdb/ChangeLog:
2017-10-04  Pedro Alves  <palves@redhat.com>

	* remote.c (get_remote_arch_state): New 'gdbarch' parameter.  Use
	it instead of target_gdbarch.
	(get_remote_state, get_remote_packet_size): Adjust
	get_remote_arch_state calls, passing down target_gdbarch
	explicitly.
	(packet_reg_from_regnum, packet_reg_from_pnum): New parameter
	'gdbarch' and use it instead of target_gdbarch.
	(get_memory_packet_size): Adjust get_remote_arch_state calls,
	passing down target_gdbarch explicitly.
	(struct stop_reply) <arch>: New field.
	(remote_parse_stop_reply): Use the stopped thread's architecture,
	not the current inferior's.  Save the architecture in the
	stop_reply.
	(process_stop_reply): Use the stop reply's architecture.
	(process_g_packet, remote_fetch_registers)
	(remote_prepare_to_store, store_registers_using_G)
	(remote_store_registers): Adjust get_remote_arch_state calls,
	using the regcache's architecture.
	(remote_get_trace_status): Adjust get_remote_arch_state calls,
	passing down target_gdbarch explicitly.
	* spu-multiarch.c (spu_thread_architecture): Defer to the target
	beneath instead of calling target_gdbarch.
	* target.c (default_thread_architecture): Use the specified
	inferior's architecture, instead of the current inferior's
	architecture (via target_gdbarch).

gdb/testsuite/ChangeLog:
2017-10-04  Pedro Alves  <palves@redhat.com>

	* gdb.multi/hangout.c: Include <unistd.h>.
	(hangout_loop): New function.
	(main): Call alarm.  Call hangout_loop in a loop.
	* gdb.multi/hello.c: Include <unistd.h>.
	(hello_loop): New function.
	(main): Call alarm.  Call hangout_loop in a loop.
	* gdb.multi/multi-arch.exp: Test running to a breakpoint one
	inferior with the other selected.