[gas][arm] Set context table for '.arch_extension'

This patch fixes .arch_extension behaviour.

Currently, context table for '.arch_extension' is only set while
"-march" processing, but it is not set while .arch processing,
so following code is rejected

  .syntax unified
  .thumb
  .arch armv8.1-m.main
  .arch_extension mve.fp

  unless -march=armv8.1-m.main is given.

Committing on behalf of Vladimir Murzin

gas/ChangeLog:
2019-12-10  Vladimir Murzin  <vladimir.murzin@arm.com>

	* config/tc-arm.c (s_arm_arch): Set selected_ctx_ext_table.
	* testsuite/gas/arm/mve-arch-ext.s: New.
	* testsuite/gas/arm/mve-arch-ext.d: New.

gas/ChangeLog

@@ -1,3 +1,9 @@
+2019-12-10  Vladimir Murzin  <vladimir.murzin@arm.com>
+
+	* config/tc-arm.c (s_arm_arch): Set selected_ctx_ext_table.
+	* testsuite/gas/arm/mve-arch-ext.s: New.
+	* testsuite/gas/arm/mve-arch-ext.d: New.
+
 2019-12-09  Jan Beulich  <jbeulich@suse.com>
 
 	* config/tc-i386-intel.c (O_oword_ptr): Move.

gas/config/tc-arm.c

@@ -32633,6 +32633,7 @@ s_arm_arch (int ignored ATTRIBUTE_UNUSED)
     if (streq (opt->name, name))
       {
 	selected_arch = opt->value;
+	selected_ctx_ext_table = opt->ext_table;
 	selected_ext = arm_arch_none;
 	selected_cpu = selected_arch;
 	strcpy (selected_cpu_name, opt->name);

gas/testsuite/gas/arm/mve-arch-ext.d

@@ -0,0 +1,7 @@
+# name: MVE context sensitive .arch_extension under no -march
+# objdump: -dr --prefix-addresses --show-raw-insn -marmv8.1-m.main
+
+.*: +file format .*arm.*
+
+Disassembly of section .text:
+0[0-9a-f]+ <[^>]+> ea52 136f 	asrl	r2, r3, #5

gas/testsuite/gas/arm/mve-arch-ext.s

@@ -0,0 +1,6 @@
+.syntax unified
+.thumb
+.arch armv8.1-m.main
+.arch_extension mve
+
+asrl	r2, r3, #5