py/asmthumb: Flush D-cache, and invalidate I-cache on STM32F7.

Tested on a STM32F7DISCO at 216MHz.  All tests generating code (inlineasm,
native, viper) now pass, except pybnative/while.py, but that's because
there is no LED(2).

py/asmthumb.c

@@ -90,6 +90,15 @@ void asm_thumb_start_pass(asm_thumb_t *as, uint pass) {
 void asm_thumb_end_pass(asm_thumb_t *as) {
     (void)as;
     // could check labels are resolved...
+
+    #if defined(MCU_SERIES_F7)
+    if (as->pass == ASM_THUMB_PASS_EMIT) {
+        // flush D-cache, so the code emited is stored in memory
+        SCB_CleanDCache_by_Addr((uint32_t*)as->code_base, as->code_size);
+        // invalidate I-cache
+        SCB_InvalidateICache();
+    }
+    #endif
 }
 
 // all functions must go through this one to emit bytes