tci: move tci_regs to tcg_qemu_tb_exec's stack

Groundwork for supporting multiple TCG contexts.

Compile-tested for all targets on an x86_64 host.
Suggested-by: NRichard Henderson <rth@twiddle.net>
Acked-by: NRichard Henderson <rth@twiddle.net>
Signed-off-by: NEmilio G. Cota <cota@braap.org>
Signed-off-by: NRichard Henderson <richard.henderson@linaro.org>