gdb: Deprecate MPX commands.

This patch deprecates the MPX commands "show/set mpx bound".
Intel listed Intel(R) Memory Protection Extensions (MPX) as removed
in 2019.  Following gcc v9.1, the linux kernel v5.6 and glibc v2.35,
deprecate MPX in GDB.

gdb/NEWS

@@ -3,6 +3,9 @@
 
 *** Changes since GDB 14
 
+* The MPX commands "show/set mpx bound" have been deprecated, as Intel
+  listed MPX as removed in 2019.
+
 * Building GDB and GDBserver now requires a C++17 compiler.
   For example, GCC 9 or later.
 

gdb/doc/gdb.texinfo

@@ -26652,6 +26652,9 @@ whose bounds are to be changed, @var{lbound} and @var{ubound} are new values
 for lower and upper bounds respectively.
 @end table
 
+Both commands are deprecated and will be removed in future versions of
+@value{GDBN}.  MPX itself was listed as removed by Intel in 2019.
+
 When you call an inferior function on an Intel MPX enabled program,
 GDB sets the inferior's bound registers to the init (disabled) state
 before calling the function.  As a consequence, bounds checks for the

gdb/i386-tdep.c

@@ -9132,17 +9132,19 @@ is \"default\"."),
 
   /* Add "bound" command for the show mpx commands list.  */
 
-  add_cmd ("bound", no_class, i386_mpx_info_bounds,
+  cmd_list_element *c = add_cmd ("bound", no_class, i386_mpx_info_bounds,
 	   "Show the memory bounds for a given array/pointer storage\
  in the bound table.",
 	   &mpx_show_cmdlist);
+  deprecate_cmd (c, nullptr);
 
   /* Add "bound" command for the set mpx commands list.  */
 
-  add_cmd ("bound", no_class, i386_mpx_set_bounds,
+  c = add_cmd ("bound", no_class, i386_mpx_set_bounds,
 	   "Set the memory bounds for a given array/pointer storage\
  in the bound table.",
 	   &mpx_set_cmdlist);
+  deprecate_cmd (c, nullptr);
 
   gdbarch_register_osabi (bfd_arch_i386, 0, GDB_OSABI_SVR4,
 			  i386_svr4_init_abi);