Multibyte register C/C++ export

Currently only word sizes are supported; 8, 16, 32, 64 bits. Need to support arbitrarily sized registers.

If a register is an integer multiple of a word size, use the largest word size that can span the register for efficient access.

eg.

• 16 bytes => 2x 64 bit words
• 17 bytes => 17x bytes
• 14 bytes => 7x 16 bit words
• 12 bytes => 3x 32 bit words