fix: detect avx2 in base64

[equal-l2]

Currently, bun doesn't even build on non-AVX2 environment.
some package managers intentionally disable AVX2 to support older systems.

This PR fixes the build on non-AVX2 env, though I don't assure bun runs properly.

Disclaimer: I have not tested this because I don't have a non-AVX2 machine.

[equal-l2]

Would anyone approve the workflows?

[KevinBLT]

I would also need a non-AVX2 build 👍

[memothelemo]

There's also a file that fails to compile on my non-AVX2 machine (src/base64/fastavxbase64.c).

You can add this condition as well if it has AVX2 capability. ;-)

[equal-l2]

@memothelemo Thank you for pointing it out. I added a check.
Would you try again?

[equal-l2]

@oven-sh Could you approve running workflows?
I can't build bun on my local env at this moment.

[memothelemo]

@equal-l2 I was able to compile Base64 with some modifications with the header files before I send a comment to you.

I do not quite sure if it would compile the entire bun program successfully.
I'm still on my way to test your PR repository. ;)

[equal-l2]

The same change has been made and merged in #824, which is contained in the new release 0.1.5.
This should enable you to build bun from source on non-AVX2 machines.

Closing now.