Jetson TK1, the Nvidia dev board with 32-bit Tegra K1 has JTAG. The 64-bit chip has a completely different instruction set of course, is that the problem?
The 64-bit Tegra K1 doesn't run ARMv8 code natively. It's a proprietary VLIW architecture with an undocumented (AFAIK) ISA. It executes ARMv8 using a firmware-level dynamic binary translator, similarly to how Transmeta processors were running x86 code.
Not ewww, it's one of the more interesting developments (well, technically a revival) in computer architecture from the industry. I just wish they'd 1) document their architecture and 2) allow access third party access to the bare metal.
Well, I don't think it's valid to call that non-native, given that it has no other mode of operation. x86 hasn't been "native" for a long time either, since it's microcoded at the very least (although not to the point of doing BT perhaps).