Support i386 option for absolute baseline x86 support

[Keno]

Fixes #6715. @nalimilan Does this work for you?

[nalimilan]

Cool! I thought it would have required many more changes.

I can't try it myself since I only have an i5 CPU, but I'll ask the Fedora user who tested it. I guess if developers are OK you should just merge the PR, I'll confirm later if it works.

[ViralBShah]

This is useful to have for Debian too, and distros should probably build with i386, right? Is this something that can be set when Julia is started? It's ok if the julia binaries are i386, but generated code should be native.

Cc: @sebastien-villemot

[Keno]

No, this needs to be set at compile time and needs to be the same for the jitted code since llvm switches calling conventions on us depending on what CPU feature are available.

[JeffBezanson]

Distros vary in their policies; some specifically aim to optimize code for modern cpus.

[Keno]

Ok, I'll merge this, since the main point of this was to let people know if was happening (plus my julia repos are so far off master, that I need to push everything to a branch first to make sure I don't accidentally commit something I shouldn't ;) )

[nalimilan]

Oh, and for x86_64 builds, should distributions also pass i386 and it will be understood correctly (i.e. SSE and SSE2 can still be enabled)? Or should an x86_64 target be also added? Sorry, I don't know where to look to see a list of targets accepted by LLVM.

[nalimilan]

@Keno Ideas? ^

[Keno]

No in theory it should just work. I do seem to recall however that least extensive x86_64 has all the core2 features, but I may be wrong there.

[nalimilan]

@Keno OK, will try. But Core 2 CPUs support e.g. SSE3 and SSSE3, contrary to older x86_64 CPUs.