Fix support for building with --static-swift-stdlib for armv7

[xtremekforever]

The atomic library is used in the Swift stdlib for 32-bit architectures. As such, when attempting to compile a binary containing the static Swift stdlib using a Swift SDK for armv7, -latomic must be linked. This is not needed for regular cross-compilation, just when passing --static-swift-stdlib.

I've added -latomic for "armv7" and added a test case in LinuxRecipeTests to ensure that it is included in the toolset.json file.

Now, I can build my binary that contains the statically linked Swift stdlib with this command:

swift build --swift-sdk 6.0.3-RELEASE_debian_bookworm_armv7 --static-swift-stdlib

This generates a rather large binary when also linking other things such as Foundation and Dispatch, as is to be expected:

$ pwd
~/tmp/hummingbird-examples/hello
$ du -hs .build/debug/App
123M	.build/debug/App

But that's fine, because it works perfectly on the target (Raspberry Pi 2) and provides yet another option for compiling binaries using the Swift SDK.

[MaxDesiatov]

LGTM, modulo the i686 nit

[MaxDesiatov]

@swift-ci test