target/arm: Enable SCTLR_EL1.BT0 for aarch64-linux-user

target/arm: Enable SCTLR_EL1.BT0 for aarch64-linux-user

This controls whether the PACI{A,B}SP instructions trap with BTYPE=3
(indirect branch from register other than x16/x17).  The linux kernel
sets this in bti_enable().

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/998
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20220427042312.294300-1-richard.henderson@linaro.org
[PMM: remove stray change to makefile comment]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>