Let 32-bit x86 devices report i386 architecture

What does this MR do?

• Sets reporting of i386 architecture for other 32-bit x86 hardware (i586, and i686, that are likely encountered and might work, requiring MMX instruction set)
• Sets reporting of i386 on devices when the go runtime's architecture is queried and GOARCH is 386, to match the above step

Why was this MR needed?

To be able to use the same gitlab-runner-helper image across all 32-bit x86 devices, similar to how ARM devices reuse their helper image by reporting arm architecture across the variants.

This is a companion piece for !1157 (closed).

An example of an i686 device is the Intel Edison, an example for i586 is Siemens IOT2000 (not a good example as it's i586, but doesn't have the required MMX instructions to run the compiled gitlab runner, though hardware might be still relevant.)

Are there points in the code the reviewer needs to double check?

Does this MR meet the acceptance criteria?

• Documentation created/updated
• Added tests for this feature/bug
• In case of conflicts with master - branch was rebased

What are the relevant issue numbers?

#3919 (closed)