aarch64: Allow testing for the presence of FEAT_AFP

Allows querying for the existence of FEAT_AFP (alternate floating-point
behavior).

While we're in the same area, we can also fill in the remaining defined
bits.
diff --git a/src/cpu-features.h b/src/cpu-features.h
index b947466..2241693 100644
--- a/src/cpu-features.h
+++ b/src/cpu-features.h
@@ -177,7 +177,9 @@
   V(kSVESHA3,             "SVE SHA3",               "svesha3")                 \
   V(kSVEBitPerm,          "SVE BitPerm",            "svebitperm")              \
   V(kSVEAES,              "SVE AES",                "sveaes")                  \
-  V(kSVEPmull128,         "SVE Pmull128",           "svepmull")
+  V(kSVEPmull128,         "SVE Pmull128",           "svepmull")                \
+  /* Alternate floating-point behavior                                      */ \
+  V(kAFP,                 "AFP",                    "afp")
 // clang-format on