[6.3] CMake Android: ensure _Builtin_float is built before _math
_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
(cherry picked from commit 817a890a672256bd3b95e1b73a6ccf1e96c63f96)
