armv7m: Allow SHCSR writes to change pending and active bits

Implement the NVIC SHCSR write behaviour which allows pending and
active status of some exceptions to be changed.
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>
Reviewed-by: NAlex Bennée <alex.bennee@linaro.org>

hw/intc/armv7m_nvic.c

@@ -755,8 +755,17 @@ static void nvic_writel(NVICState *s, uint32_t offset, uint32_t value)
         cpu->env.v7m.ccr = value;
         break;
     case 0xd24: /* System Handler Control.  */
-        /* TODO: Real hardware allows you to set/clear the active bits
-           under some circumstances.  We don't implement this.  */
+        s->vectors[ARMV7M_EXCP_MEM].active = (value & (1 << 0)) != 0;
+        s->vectors[ARMV7M_EXCP_BUS].active = (value & (1 << 1)) != 0;
+        s->vectors[ARMV7M_EXCP_USAGE].active = (value & (1 << 3)) != 0;
+        s->vectors[ARMV7M_EXCP_SVC].active = (value & (1 << 7)) != 0;
+        s->vectors[ARMV7M_EXCP_DEBUG].active = (value & (1 << 8)) != 0;
+        s->vectors[ARMV7M_EXCP_PENDSV].active = (value & (1 << 10)) != 0;
+        s->vectors[ARMV7M_EXCP_SYSTICK].active = (value & (1 << 11)) != 0;
+        s->vectors[ARMV7M_EXCP_USAGE].pending = (value & (1 << 12)) != 0;
+        s->vectors[ARMV7M_EXCP_MEM].pending = (value & (1 << 13)) != 0;
+        s->vectors[ARMV7M_EXCP_BUS].pending = (value & (1 << 14)) != 0;
+        s->vectors[ARMV7M_EXCP_SVC].pending = (value & (1 << 15)) != 0;
         s->vectors[ARMV7M_EXCP_MEM].enabled = (value & (1 << 16)) != 0;
         s->vectors[ARMV7M_EXCP_BUS].enabled = (value & (1 << 17)) != 0;
         s->vectors[ARMV7M_EXCP_USAGE].enabled = (value & (1 << 18)) != 0;