stdlib: check for ARM/ARM64/AArch64 more thoroughly

[futurejones]

Changes needed for successful builds of 4.2 for AArch64 on the Swift Community CI.
(cherry picked from #19462)

Resolves SR-NNNN.

[compnerd]

@swift-ci please test

[compnerd]

@airspeedswift - seems like @futurejones would like to nominate this for the 4.2-branch. This impacts the swift standard library, but should only impact the non-Apple ARM/AArch64 build targets, and is simply enabling the same paths under different names, so I consider it a low risk change.

[bob-wilson]

We generally don't add enhancements like this on old release branches. We should focus new work on the master branch and the active release branches. Besides, why do we need CI for a branch that isn't changing anymore?

[futurejones]

@bob-wilson

1. As far as I am aware the swift-4.2-branch is the "current" release branch and defiantly not an "old" release branch. It is also "active" as we can see from the recent 4.2.1 release.
2. This is not an enhancement, it is a fix for a breaking change that was introduced in 4.2 that is preventing successful builds on AArch64 systems.
3. The CI you are referring to is generously setup and maintained by members of the Swift open-source community.

In Swift 4.1 we saw 4.1, 4.1.1, 4.1.2 and 4.1.3 releases. There absolutely no reason why we can not see a similar pattern of releases for Swift 4.2.

Also these changes have already been added to the master - #19462

[bob-wilson]

We're not planning any more releases from the 4.2 branch. The current release branch is for Swift 5.0, and I think we should focus our energies on that branch.

[futurejones]

@bob-wilson as stated before - these changes have already been added in PR 19462
There is no reason not to allow this to be added to swift-4.2-branch as well.

[hpux735]

This puts the Swift Arm community in a position again to maintain parallel repos. We've tried to work with the core team about this over the years, and they've expressed interest in helping us stay on mainline. However, not being willing to work with us pushes us away again.

Swift 5 has not been released, and it's not usable for any user in production settings, let alone on Arm64. Saying that we should accept the fact that Swift 4.2 doesn't work on Arm64, and never will, because we need to wait until some indeterminate point in the future (WWDC 2019?), when Swift 5 is released is pretty painful.

If you look at the changes proposed, they are incredibly minor. Furthermore, master has diverged from this enough that this patch would not be able to be cherry-picked from master into this branch.

[futurejones]

@weissi @kevints is it possible to get this into the 4.2.3 Release?
Thanks.

[weissi]

@tkremenek / @rjmccall what are your thoughts on this one? It looks okay to me

[rjmccall]

LGTM.

[weissi]

thanks @rjmccall , will take this one then

[futurejones]

Thank you.