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ARMv4T has BX as its only interworking instruction. In order to support testing of different architecture revisions with a qemu binary that may have been built for, say ARMv6T2, fill in the blank required to make calls to helpers in thumb mode. Reviewed-by: Peter Maydell <peter.maydell@linaro.org> Signed-off-by: Richard Henderson <richard.henderson@linaro.org>