improve aarch64 feature gate in rust bindings (#2069)

Cargo.toml

@@ -45,7 +45,9 @@ dynamic_linkage = []
 arch_all = ["arch_x86", "arch_arm", "arch_aarch64", "arch_riscv", "arch_mips", "arch_sparc", "arch_m68k", "arch_ppc", "arch_s390x", "arch_tricore"]
 arch_x86 = []
 arch_arm = []
-arch_aarch64 = []
+# NOTE: unicorn-c only separates on top-level arch name,
+#       not on the bit-length, so we include both arm and aarch64
+arch_aarch64 = ["arch_arm"]
 arch_riscv = []
 arch_mips = []
 arch_sparc = []