target/arm: Split out rebuild_hflags_a64

target/arm: Split out rebuild_hflags_a64

Create a function to compute the values of the TBFLAG_A64 bits
that will be cached.  For now, the env->hflags variable is not
used, and the results are fed back to cpu_get_tb_cpu_state.

Note that not all BTI related flags are cached, so we have to
test the BTI feature twice -- once for those bits moved out to
rebuild_hflags_a64 and once for those bits that remain in
cpu_get_tb_cpu_state.

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20191023150057.25731-3-richard.henderson@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>