[clang] [clang] Enable --print-supported-extensions for all targets (PR #66586)

[David Spickett via cfe-commits]

DavidSpickett wrote:

One way to check the formats is to add checks for the column titles (if any) and first extension listed in `clang/test/Driver/print-supported-extensions.c`, to document the existing behaviour. RISC-V calls into it's own ABI info class which only has unit tests so if you change things here we won't notice with the current tests.

Then you can build this change on top of that, and potentially change RISC-Vs format if there is agreement there.

https://github.com/llvm/llvm-project/pull/66586