target/arm: Add lazy-FP-stacking support to v7m_stack_write()

target/arm: Add lazy-FP-stacking support to v7m_stack_write()

Pushing registers to the stack for v7M needs to handle three cases:
 * the "normal" case where we pend exceptions
 * an "ignore faults" case where we set FSR bits but
   do not pend exceptions (this is used when we are
   handling some kinds of derived exception on exception entry)
 * a "lazy FP stacking" case, where different FSR bits
   are set and the exception is pended differently

Implement this by changing the existing flag argument that
tells us whether to ignore faults or not into an enum that
specifies which of the 3 modes we should handle.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20190416125744.27770-23-peter.maydell@linaro.org