Bugfix
   * Fix clang and armclang compilation error when targeting certain Arm
     M-class CPUs (Cortex-M0, Cortex-M0+, Cortex-M1, Cortex-M23,
     SecurCore SC000). Fixes #1077.
Changes
   * Enable Arm / Thumb bignum assembly for most Arm platforms when
     compiling with gcc, clang or armclang and -O0.