OS, compiler minimum versions and minimum processor requirements for running Julia

[ViralBShah]

It's been a while since we bumped minimum OS requirements and compiler versions.

OSes for running:

• For Windows, pick Windows 10, since Windows 7 is no longer supported by Microsoft. https://support.microsoft.com/en-in/help/4057281/windows-7-support-ended-on-january-14-2020 (Drop Windows 7 official support #35974)
• For Mac, what are the right considerations? Is there a benefit to bumping the min supported version? Even if just to simplify our build process? I suspect Mac should continue with 10.8.
• For Linux, we say kernel 2.6.18+. That sounds specific enough.

CPUs for running:

• Define clearly the oldest processor we support. (E.g. Julia 1.4 fails on startup (AMD Phenom on Linux) #35215 for AMD phenom pentium 4 for x86)
• Pick something similar for an equivalent Intel processor

For building:

• For compiling Julia, LLVM 9 leads to a new lower bound on gcc - julia fails to build #34555 (comment)

[ViralBShah]

Also some of the installation and compilation related documentation in docs/build should probably be shown more prominently, perhaps in the manual.

[PallHaraldsson]

[EDIT: if this is just to build, not run Julia, just ignore my comment]

pick Windows 10, since Windows 7 is no longer supported

Is there a real need? 2/3 are on Windows 10, and the rest more or less on Windows 7.

I'm not using Windows so don't care much personally, just seems premature. Next LTS will probably be Julia 1.5 right? Change after that?

[musm]

[EDIT: if this is just to build, not run Julia, just ignore my comment]

pick Windows 10, since Windows 7 is no longer supported

Is there a real need? 2/3 are on Windows 10, and the rest more or less on Windows 7.

I'm not using Windows so don't care much personally, just seems premature. Next LTS will probably be Julia 1.5 right? Change after that?

Julia being not supported on Windows 7, doesn't mean it'll stop working on Windows 7, just that it won't be supported by the Julia language team.

[ViralBShah]

At the moment it basically means that we can make assumptions about some minimum capabilities available by default - a reasonable version of powershell, maybe the windows terminal, etc. For Windows 7 users, they'll probably have to install some extra things or upgrades.

[ViralBShah]

#36123 updates the build dependencies

[ViralBShah]

Based on what @staticfloat said in #35215, we'll adopt the Core 2 instruction set as the minimum for Intel and Bulldozer for AMD.

Is there something we should do for ARM?

[yuyichao]

If anything that should at Most be what the official binary is compiled for and should not be the required feature. And there's also very little point of raising that requirement since basically no performance sensitive code is affected.

Nothing should change for arm.

[ViralBShah]

It is the required minimum CPU - in that older than those may not work - as discussed in #35215. This is documenting the current practice -not setting a policy for a new practice.

[ViralBShah]

@yuyichao Clearly our binaries do not work on armv6 processors. So presumably there is something we need to say about what works and what does not.