Nimble won't compile on Arm Linux

[younata]

For all... probably 0 people running Nimble on a raspberry pi..
For those running swift in a docker container on an apple silicon mac.

Discovered while doing #980, basically, if you try to run swift test on an Arm box, you get a bunch of build errors like the following:

Googling around, it seems like this is just a register or something that isn't in the arm version of GLIBC?

We can definitely avoid this by wrapping our throwAssertion matcher around an #if canImport(Darwin) || arch(x86_64) compilation block. But, I'd love an actual fix for this if we can do that.

[younata]

Bumping this to the next (major?) release in favor of getting async/await support out the door sooner.

[sliemeobn]

For all... probably 0 people running Nimble on a raspberry pi.

just FYI, the same issues occur when building on an Apple Silicon mac.
so, anyone doing server-side swift who wants to run their tests in a linux container (eg: vscode dev container) cannot use Nimble ATM.