SDK portability to IAR toolchain

[bavison]

The SDK is a useful basis for Pico projects built using toolchains other than GCC, but it currently relies upon a number of GCC-only features. The IAR toolchain is one of those that is most divergent from GCC (certainly compared to the likes of Clang) but nevertheless it looks practical to extend the SDK so that it can be built with either GCC or IAR.

I'm creating this issue as a reference point for associated PRs (the changes are quite substantial, so I think it makes sense to split them into multiple PRs).