Apply 'const' in more places

Many global arrays in gdb could be marked "const" but are not.  This
patch changes some of them.  (There may be other arrays that could
benefit from this treatment.  I only examined arrays of strings.)

This lets the linker move some symbols to the readonly data section.
For example, previously:

0000000000000000 d _ZL18can_use_agent_enum

is now:

0000000000000030 r _ZL18can_use_agent_enum

2020-09-14  Tom Tromey  <tromey@adacore.com>

	* x86-tdep.h (x86_in_indirect_branch_thunk): Update.
	* x86-tdep.c (x86_is_thunk_register_name)
	(x86_in_indirect_branch_thunk): Update.
	* sparc64-tdep.c (sparc64_fpu_register_names)
	(sparc64_cp0_register_names, sparc64_register_names)
	(sparc64_pseudo_register_names): Now const.
	* sparc-tdep.h (struct gdbarch_tdep) <fpu_register_names,
	cp0_registers_num>: Now const.
	* sparc-tdep.c (sparc_core_register_names)
	(sparc32_fpu_register_names, sparc32_cp0_register_names)
	(sparc32_pseudo_register_names): Now const.
	(validate_tdesc_registers): Update.
	* rust-lang.c (rust_extensions): Now const.
	* p-lang.c (p_extensions): Now const.
	* objc-lang.c (objc_extensions): Now const.
	* nto-tdep.c (nto_thread_state_str): Now const.
	* moxie-tdep.c (moxie_register_names): Now const.
	* mips-tdep.h (struct gdbarch_tdep) <mips_processor_reg_names>:
	Now const.
	* mips-tdep.c (mips_generic_reg_names, mips_tx39_reg_names)
	(mips_linux_reg_names): Now const.
	(mips_gdbarch_init): Update.
	* microblaze-tdep.c (microblaze_register_names): Now const.
	* m68k-tdep.c (m68k_register_names): Now const.
	* m32r-tdep.c (m32r_register_names): Now const.
	* ia64-tdep.c (ia64_register_names): Now const.
	* i386-tdep.h (struct gdbarch_tdep) <register_names,
	ymmh_register_names, ymm16h_regnum, mpx_register_names,
	k_register_names, zmmh_register_names, xmm_avx512_register_names,
	ymm_avx512_register_names, pkeys_register_names>: Now const.
	* i386-tdep.c (i386_register_names, i386_zmm_names)
	(i386_zmmh_names, i386_k_names, i386_ymm_names, i386_ymmh_names)
	(i386_mpx_names, i386_pkeys_names, i386_bnd_names)
	(i386_mmx_names, i386_byte_names, i386_word_names): Now const.
	* f-lang.c (f_extensions): Now const.
	* d-lang.c (d_extensions): Now const.
	* csky-tdep.c (csky_register_names): Now const.
	* charset.c (default_charset_names, charset_enum): Now const.
	(_initialize_charset): Update.
	* c-lang.c (c_extensions, cplus_extensions, asm_extensions): Now
	const.
	* bsd-uthread.c (bsd_uthread_solib_names): Now const.
	(bsd_uthread_solib_loaded): Update.
	(bsd_uthread_state): Now const.
	* amd64-tdep.c (amd64_register_names, amd64_ymm_names)
	(amd64_ymm_avx512_names, amd64_ymmh_names)
	(amd64_ymmh_avx512_names, amd64_mpx_names, amd64_k_names)
	(amd64_zmmh_names, amd64_zmm_names, amd64_xmm_avx512_names)
	(amd64_pkeys_names, amd64_byte_names, amd64_word_names)
	(amd64_dword_names): Now const.
	* agent.c (can_use_agent_enum): Now const.
	* ada-tasks.c (task_states, long_task_states): Now const.
	* ada-lang.c (known_runtime_file_name_patterns)
	(known_auxiliary_function_name_patterns, attribute_names)
	(standard_exc, ada_extensions): Now const.

gdbserver/ChangeLog
2020-09-14  Tom Tromey  <tromey@adacore.com>

	* tracepoint.cc (eval_result_names): Now const.
	* ax.cc (gdb_agent_op_names): Now const.

gdb/charset.c

@@ -271,13 +271,13 @@ show_target_wide_charset_name (struct ui_file *file,
 		      value);
 }
 
-static const char *default_charset_names[] =
+static const char * const default_charset_names[] =
 {
   DEFAULT_CHARSET_NAMES
   0
 };
 
-static const char **charset_enum;
+static const char * const *charset_enum;
 
 
 /* If the target wide character set has big- or little-endian
@@ -1004,7 +1004,7 @@ _initialize_charset ()
   find_charset_names ();
 
   if (charsets.charsets.size () > 1)
-    charset_enum = (const char **) charsets.charsets.data ();
+    charset_enum = (const char * const *) charsets.charsets.data ();
   else
     charset_enum = default_charset_names;