target/arm: Clear exclusive monitor on v7M reset, exception entry/exit

target/arm: Clear exclusive monitor on v7M reset, exception entry/exit

For M profile we must clear the exclusive monitor on reset, exception
entry and exception exit.  We weren't doing any of these things; fix
this bug.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@xilinx.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 1505137930-13255-3-git-send-email-peter.maydell@linaro.org