cpu/esp32: fix problem with tests/thread_race
cpu/esp32: fix problem with tests/thread_race

The problem seemed to be a pipelining problem of write and read instructions when swapping the context. An isync instruction when returning from a context switch solves the potential pipelining problem.
authmillenon/RIOT,biboc/RIOT,OTAkeys/RIOT,cladmi/RIOT,gebart/RIOT,aeneby/RIOT,mfrey/RIOT,x3ro/RIOT,OTAkeys/RIOT,mfrey/RIOT,josephnoir/RIOT,BytesGalore/RIOT,kYc0o/RIOT,authmillenon/RIOT,miri64/RIOT,x3ro/RIOT,OTAkeys/RIOT,kaspar030/RIOT,smlng/RIOT,cladmi/RIOT,OlegHahm/RIOT,BytesGalore/RIOT,toonst/RIOT,ant9000/RIOT,OlegHahm/RIOT,gebart/RIOT,basilfx/RIOT,yogo1212/RIOT,x3ro/RIOT,yogo1212/RIOT,RIOT-OS/RIOT,gebart/RIOT,toonst/RIOT,smlng/RIOT,kYc0o/RIOT,authmillenon/RIOT,mtausig/RIOT,gebart/RIOT,x3ro/RIOT,A-Paul/RIOT,mtausig/RIOT,biboc/RIOT,miri64/RIOT,A-Paul/RIOT,basilfx/RIOT,rfuentess/RIOT,OTAkeys/RIOT,yogo1212/RIOT,basilfx/RIOT,jasonatran/RIOT,jasonatran/RIOT,lazytech-org/RIOT,kaspar030/RIOT,A-Paul/RIOT,BytesGalore/RIOT,biboc/RIOT,kaspar030/RIOT,josephnoir/RIOT,toonst/RIOT,mfrey/RIOT,yogo1212/RIOT,A-Paul/RIOT,gebart/RIOT,smlng/RIOT,jasonatran/RIOT,rfuentess/RIOT,biboc/RIOT,josephnoir/RIOT,RIOT-OS/RIOT,miri64/RIOT,kYc0o/RIOT,OlegHahm/RIOT,toonst/RIOT,josephnoir/RIOT,miri64/RIOT,ant9000/RIOT,mtausig/RIOT,miri64/RIOT,rfuentess/RIOT,x3ro/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,authmillenon/RIOT,ant9000/RIOT,aeneby/RIOT,BytesGalore/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,cladmi/RIOT,lazytech-org/RIOT,BytesGalore/RIOT,mfrey/RIOT,authmillenon/RIOT,lazytech-org/RIOT,mtausig/RIOT,kaspar030/RIOT,OTAkeys/RIOT,mtausig/RIOT,yogo1212/RIOT,aeneby/RIOT,yogo1212/RIOT,biboc/RIOT,aeneby/RIOT,josephnoir/RIOT,jasonatran/RIOT,cladmi/RIOT,mfrey/RIOT,smlng/RIOT,authmillenon/RIOT,smlng/RIOT,A-Paul/RIOT,rfuentess/RIOT,OlegHahm/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,toonst/RIOT,basilfx/RIOT,kYc0o/RIOT,basilfx/RIOT,OlegHahm/RIOT,aeneby/RIOT,ant9000/RIOT,cladmi/RIOT,lazytech-org/RIOT,lazytech-org/RIOT,ant9000/RIOT,rfuentess/RIOT