gcc-style __asm__

Is gcc style inline assembly only supported in x86 as reviewed here: https://reviews.llvm.org/D36371

or is it supported on other platforms as well?

If so, where is that documented?

No, GCC style inline assembly is supported everywhere. However, while, Clang aims for compatibility with GCC and GNU assembler idioms, but it does not always succeed. Set your expectations accordingly.