gdbserver: Fix qSupported:xmlRegisters=i386;UnknownFeature+ handling

The target_process_qsupported method is called for each qSupported
feature that the common code does not recognize.  The only current
implementation, for x86 Linux (x86_linux_process_qsupported), assumes
that it either is called with the "xmlRegisters=i386" feature, or that
it is isn't called at all, indicating the connected GDB predates x86
XML descriptions.

That's a bad assumption however.  If GDB sends in a new/unknown (to
core gdbserver) feature after "xmlRegisters=i386", say, something like
qSupported:xmlRegisters=i386;UnknownFeature+, then when
target_process_qsupported is called for "UnknownFeature+",
x86_linux_process_qsupported clears the 'use_xml' global and calls
x86_linux_update_xmltarget, and gdbserver ends up _not_ reporting a
XML description...

This commit changes the target_process_qsupported API to instead pass
down a vector of unprocessed qSupported features in one go.

(There's an early call to target_process_qsupported(NULL) that
indicates "starting qSupported processing".  There's no matching call
to mark the end of processing, though.  I first fixed this by passing
(char *)-1 to indicate that, and adjusted the x86 backend to only
clear 'use_xml' when qSupported processing starts, and then only call
x86_linux_update_xmltarget() when (char *)-1 was passed.  However, I
wasn't that happy with the hack and came up this alternative version.)

gdb/gdbserver/ChangeLog:
2015-11-19  Pedro Alves  <palves@redhat.com>

	* linux-low.c (linux_process_qsupported): Change prototype.
	Adjust.
	* linux-low.h (struct linux_target_ops) <process_qsupported>:
	Change prototype.
	* linux-x86-low.c (x86_linux_process_qsupported): Change prototype
	and adjust to loop over all features.
	* server.c (handle_query) <qSupported>: Adjust to call
	target_process_qsupported once, passing it a vector of unprocessed
	features.
	* target.h (struct target_ops) <process_qsupported>: Change
	prototype.
	(target_process_qsupported): Adjust.