Enable 64-bit compilation with ARM Compiler 6

This patch fixes the conditional preprocessor directives in
include/mbedtls/bignum.h to enable 64-bit compilation with ARM
Compiler 6.

ChangeLog

diff --git a/ChangeLog b/ChangeLog
index ed00182..6f902fa 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -27,6 +27,8 @@
      Found by redplait #590
    * Add MBEDTLS_MPI_CHK to check for error value of mbedtls_mpi_fill_random.
      Reported and fix suggested by guidovranken in #740
+   * Fix conditional preprocessor directives in bignum.h to enable 64-bit
+     compilation when using ARM Compiler 6.
 
 Security
    * Fix authentication bypass in SSL/TLS: when auth_mode is set to optional,