repr(C) on enums is unsound on hexagon-unknown-linux-gnu

[mahkoh]

Enums on hexagon-unknown-linux-gnu are packed by default: https://godbolt.org/z/q49s5v

This is not implemented in rustc atm

rust/compiler/rustc_middle/src/ty/layout.rs

Lines 131 to 140 in b86674e

	if repr.c() {
	match &tcx.sess.target.arch[..] {
	// WARNING: the ARM EABI has two variants; the one corresponding
	// to `at_least == I32` appears to be used on Linux and NetBSD,
	// but some systems may use the variant corresponding to no
	// lower bound. However, we don't run on those yet...?
	"arm" => min_from_extern = Some(I32),
	_ => min_from_extern = Some(I32),
	}
	}

[RalfJung]

This looks like an incorrect ABI implementation then, but as long as Rust is self-consistent, "unsound" is the wrong term here I think?