Bugfix
   * Fix armclang compilation error when targeting certain Arm M-class CPUs
     (Cortex-M0, Cortex-M0+, Cortex-M1, Cortex-M23, SecurCore SC000). Enable
     bignum optimisations for most Arm platforms when compiling with -O0,
     (previously optimisations were not available in this case). Fixes #1077.