gdb: xtensa: don't error out when registers cannot be read

On Xtensa, some privileged registers cannot be read using 'p'
packet. So keep these marked as unavailable so GDB can still
fetch other registers instead of stopping at error.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>

Author: dcpleung

gdb/remote.c

@@ -8869,6 +8869,7 @@ remote_target::fetch_register_using_p (struct regcache *regcache,
   char *buf, *p;
   gdb_byte *regp = (gdb_byte *) alloca (register_size (gdbarch, reg->regnum));
   int i;
+  enum bfd_architecture bfd_arch = gdbarch_bfd_arch_info (gdbarch)->arch;
 
   if (m_features.packet_support (PACKET_p) == PACKET_DISABLE)
     return 0;
@@ -8893,6 +8894,15 @@ remote_target::fetch_register_using_p (struct regcache *regcache,
     case PACKET_UNKNOWN:
       return 0;
     case PACKET_ERROR:
+      /* On Xtensa, some privileged registers cannot be read using 'p'
+         packet. So keep these marked as unavailable so GDB can still
+         fetch other registers instead of stopping at error.  */
+      if (bfd_arch == bfd_arch_xtensa)
+        {
+          regcache->raw_supply (reg->regnum, NULL);
+          return 1;
+        }
+
       error (_("Could not fetch register \"%s\"; remote failure reply '%s'"),
 	     gdbarch_register_name (regcache->arch (), reg->regnum),
 	     result.err_msg ());