Make it more obvious that the `inline-asm` feature is deprecated and does nothing

[wt]

The inline-asm feature is a relic of the time before inline asm was supported in stable rust. We should do something to make it more obvious that that feature now does nothing and will be removed in a future major version.

One idea might be to at least have a message from build.rs that is output when the inline-asm feature is active is used.