aarch64: rcpc3: Add +rcpc3 architectural feature support flag

Indicating the presence of the Armv8.2-a feature adding further
support for the Release Consistency Model, the `+rcpc3' architectural
extension flag is added to the list of possible `-march' options in
Binutils, together with the necessary macro for encoding rcpc3
instructions.

include/opcode/aarch64.h

@@ -226,6 +226,8 @@ enum aarch64_feature_bit {
   AARCH64_FEATURE_SME2p1,
   /* SVE2.1 instructions.  */
   AARCH64_FEATURE_SVE2p1,
+  /* RCPC3 instructions.  */
+  AARCH64_FEATURE_RCPC3,
   AARCH64_NUM_FEATURES
 };