Enable 64-bit limbs for all Aarch64 builds.

GCC and Clang do not define __ARMCC_VERSION when building for Aarch64.
Yet they should also use 64-bit limbs for Aarch64 builds.
diff --git a/include/mbedtls/bignum.h b/include/mbedtls/bignum.h
index 0b26727..f854ca8 100644
--- a/include/mbedtls/bignum.h
+++ b/include/mbedtls/bignum.h
@@ -140,9 +140,8 @@
             typedef unsigned int mbedtls_t_udbl __attribute__((mode(TI)));
             #define MBEDTLS_HAVE_UDBL
         #endif /* !MBEDTLS_NO_UDBL_DIVISION */
-    #elif defined(__ARMCC_VERSION) && defined(__aarch64__)
+    #elif defined(__aarch64__)
         /*
-         * __ARMCC_VERSION is defined for both armcc and armclang and
          * __aarch64__ is only defined by armclang when compiling 64-bit code
          */
         #if !defined(MBEDTLS_HAVE_INT64)