I'm also getting a new Macbook pro with a M1 and I also develop React Native and iOS apps and after careful research I can safely say that with Xcode 12 you should be able to build your react native app for both chipsets.
(as per my comments) here is the snippet from Xcode's landing page -
Xcode 12 is built as a Universal app that runs 100% natively on Intel-based CPUs and Apple Silicon for great performance and a snappy interface.* It also includes a unified macOS SDK that includes all the frameworks, compilers, debuggers, and other tools you need to build apps that run natively on Apple Silicon and the Intel x86_64 CPU.
Until a month ago applications such as VSCode would not have functioned on your M1 Mac as well as several other apps, but updates have been pushed since which allow for a M1 device to more or less function like any other Mac. But the general consensus is that there might be potential issues with other 3rd party apps (unrelated to XCode or VSCode) and these might take a few months to be resolved. So all in all at this stage it would be worth it to get a new Mac, but be prepared for some bugs with 3rd party apps.
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