Assembly directives are not clear about which ones are target-specific

[ehuss]

The list of "guaranteed directives" at https://github.com/rust-lang/reference/blob/dad82335e200081ea498b0fb454b9ed3343f8cd5/src/inline-assembly.md#directives-support seems unclear to me which ones are target-specific. In the sections below, there are several target-specific directives. However, looking over the current list of non-specific ones, I see several that are also target-specific.

Some examples:

• .alt_entry — Seems to be LLVM only (!?), and Mach-O only?
• .insn — Seems to be i386 and RISC-V, but not ARM
• .inst — Seems to be ARM-only. ...etc.

I think it would be good to carefully review the list, and determine which ones are target-specific. We should also figure out what to do about things that seem to differ between LLVM and GNU.