CMake Android: ensure _Builtin_float is built before _math

[edymtt]

_math would be able to build against either the Swift or the clang module for _Builtin_float; however the build will fail if the Swift module is present but does not contain the architectures we need, with errors like

<unknown>:0: error: could not find module
'_Builtin_float' for target 'armv7-unknown-linux-android'; found:
aarch64-unknown-linux-android, x86_64-unknown-linux-android, at:
/home/build-user/build/swift-project/Ninja-Release/swift-linux-x86_64/lib/swift/android/_Builtin_float.swiftmodule/armv7-unknown-linux-android

In other words, in this situation we are not falling back to the clang module.

Addresses rdar://165768601

[edymtt]

@swift-ci please smoke test

[edymtt]

@swift-ci please build toolchain Windows

[finagolfin]

Any particular reason you are looking into this?

I don't understand how the two modules are related in the first place, as there is no indication of _Builtin_float in _math: is it just implicitly applied to every module? I thought of making this dependency explicit, as you're doing in this pull, but chose not to because I didn't see why it's required.

There are two other bugs combining with this seeming implicit dependency to cause the Android CI to flake about 10% of the time: a long-standing compiler bug in sometimes misapplying these modules from different architectures, #55637, and @jakepetroules found that we're installing the Android swiftmodules for different arches incorrectly.

Since I don't understand the implicit dependency and nobody has fixed the compiler bug for many years, I have been looking into the installation issue and think I have the reason for that: just need to pin it down and try a fix in the Android CI build script.

[finagolfin]

Another issue is that this isn't the only flaky dependency error of the same compiler bug that we're seeing, here's another one on the 6.3 CI yesterday, which this pull would not fix:

-emit-module-interface-path /home/build-user/build/swift-project/Ninja-Release/swift-linux-x86_64/./lib/swift/android/Android.swiftmodule/x86_64-unknown-linux-android.swiftinterface -emit-private-module-interface-path /home/build-user/build/swift-project/Ninja-Release/swift-linux-x86_64/./lib/swift/android/Android.swiftmodule/x86_64-unknown-linux-android.private.swiftinterface -Xfrontend -experimental-skip-non-inlinable-function-bodies @/home/build-user/build/swift-project/Ninja-Release/swift-linux-x86_64/stdlib/public/Platform/a92b8b3a7f02abacf20003726ba7f5b60c17474f.txt
22:18:30  <unknown>:0: error: could not find module '_math' for target 'x86_64-unknown-linux-android'; found: aarch64-unknown-linux-android, at: /home/build-user/build/swift-project/Ninja-Release/swift-linux-x86_64/lib/swift/android/_math.swiftmodule/x86_64-unknown-linux-android

I think fixing the installation issue is more likely to eliminate these flakes, should have a potential fix in a couple days.