C aspects of portability to IAR toolchain

[bavison]

This PR represents the bulk of the changes to C source files required to permit the SDK to build with the IAR toolchain (as verified with the pico-examples repository). Where possible, code has been refactored to use generic ISO C, but in other places I have used conditional compilation, based on either the __GNUC__ or __ICCARM__ preprocessor predefines.

I have tried to roughly group similar issues either into the same commit or into nearby commits in the history, and issues are generally ranked according to the severity of the build failure caused with IAR, from more to less serious. I'm aiming to have all the example projects building cleanly, without any warnings being emitted.

Partially addresses #1021.