Why is this happening to my software? An illegal instruction error.

[Murilo]

I've created a software which performs the Fermat's Primality 
Test, however if I input a very big number it causes an error 
saying "Illegal instruction (core dumped)". Does anyone know why?

I've used GDB and here is the message:
Program received signal SIGILL, Illegal instruction.
0x00005555555e40b0 in 
_D3std8internal4math11biguintcore7BigUint3powFNaNbNfNkMSQCcQCbQBvQBtQBjmZQs ()

Here is the link to the source code: 
https://github.com/MuriloMir/Fermat-primality-test