aarch64: Add +d128 architectural feature support

Indicating the presence of the Armv9.4-a features concerning 128-bit
Page Table Descriptors, 128-bit System Registers and Instructions,
the "+d128" architectural extension flag is added to the list of
possible -march options in Binutils, together with the necessary macro
for encoding d128 instructions.

include/opcode/aarch64.h

@@ -201,6 +201,9 @@ enum aarch64_feature_bit {
   AARCH64_FEATURE_PREDRES2,
   /* Instrumentation Extension.  */
   AARCH64_FEATURE_ITE,
+  /* 128-bit page table descriptor, system registers
+     and isntructions.  */
+  AARCH64_FEATURE_D128,
   AARCH64_NUM_FEATURES
 };