Why doesn't my processor have built-in BigInt support?

Question

As far as I understood it, BigInts are usually implemented in most programming languages as arrays containing digits, where, eg.: when adding two of them, each digit is added on

Answer 1

Seems Intel is Adding (or has added as @ time of this post - 2015) new Instructions Support for Large Integer Arithmetic.

New instructions are being introduced on Intel® Architecture Processors to enable fast implementations of large integer arithmetic. Large Integer Arithmetic is widely used in multi-precision libraries for high-performance technical computing, as well as for public key cryptography (e.g., RSA). In this paper, we describe the critical operations required in large integer arithmetic and their efficient implementations using the new instructions.

http://www.intel.com/content/www/us/en/intelligent-systems/intel-technology/ia-large-integer-arithmetic-paper.html