target/arm: Suppress debug exceptions when OS Lock set

The "OS Lock" in the Arm debug architecture is a way for software
to suppress debug exceptions while it is trying to power down
a CPU and save the state of the breakpoint and watchpoint
registers. In QEMU we implemented the support for writing
the OS Lock bit via OSLAR_EL1 and reading it via OSLSR_EL1,
but didn't implement the actual behaviour.

The required behaviour with the OS Lock set is:
 * debug exceptions (apart from BKPT insns) are suppressed
 * some MDSCR_EL1 bits allow write access to the corresponding
   EDSCR external debug status register that they shadow
   (we can ignore this because we don't implement external debug)
 * similarly with the OSECCR_EL1 which shadows the EDECCR
   (but we don't implement OSECCR_EL1 anyway)

Implement the missing behaviour of suppressing debug
exceptions.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20220630194116.3438513-4-peter.maydell@linaro.org

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