target: Add TARGET_UNAVAILABLE state

target: Add TARGET_UNAVAILABLE state

This is added for future RISC-V changes. The RISC-V debug interface can
explicitly tell a debugger when a hart is unavailable. This is used for
instance when that hart is powered down (or yet to be powered up out of
reset).

Imported from
https://github.com/riscv-collab/riscv-openocd/pull/752

Change-Id: I8a062d59eea1e5b3c788281a75159592db024683
Signed-off-by: Tim Newsome <tim@sifive.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/8911
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-by: Bernhard Rosenkränzer <bero@baylibre.com>