sme: Add support for SME

Enable SME support in gdbserver by adjusting the usual fields.  There is
not much to this patch because the code is either in gdb or it is shared
between gdbserver and gdb.  One exception is the bump to gdbserver's
PBUFSIZ from 18432 to 131104.

Since the ZA register can be quite big (256 * 256 bytes), the g/G remote
packet will also become quite big

From gdbserver/tdesc.cc:init_target_desc, I estimated the new size should
be at least (2 * 256 * 256 + 32), which yields 131104.

It is also unlikely we will find a process starting up with SVL set to 256.

Ideally we'd adjust the packet size dynamically based on what we need, but
for now this should do.

Please note we have the same limitation for SME that we have for SVE, and
that is the fact gdbserver cannot communicate vector length changes to gdb
via the remote protocol.

Thiago is working on this improvement, which hopefully will be able to be
adapted to SME in an easy way.

Co-Authored-By: Ezra Sitorus <ezra.sitorus@arm.com>
Reviewed-by: Thiago Jung Bauermann <thiago.bauermann@linaro.org>

gdbserver/server.h

@@ -104,7 +104,7 @@ extern int in_queued_stop_replies (ptid_t ptid);
 /* Buffer sizes for transferring memory, registers, etc.   Set to a constant
    value to accommodate multiple register formats.  This value must be at least
    as large as the largest register set supported by gdbserver.  */
-#define PBUFSIZ 18432
+#define PBUFSIZ 131104
 
 /* Definition for an unknown syscall, used basically in error-cases.  */
 #define UNKNOWN_SYSCALL (-1)