Makefile: Fix linker error

Instantiation of the static singleton instance thermalManager in general
requires locking, but we know its safe to do without.

Add -fno-threadsafe-statics to avoid using locking primitives, which
result in linking errors:
	undefined reference to `__cxa_guard_acquire'
	undefined reference to `__cxa_guard_release'

Signed-off-by: Stefan Brüns <stefan.bruens@rwth-aachen.de>

Marlin/Makefile

 CTUNING += -DMOTHERBOARD=${HARDWARE_MOTHERBOARD}
 endif
 #CEXTRA = -Wa,-adhlns=$(<:.c=.lst)
-CEXTRA = -fno-use-cxa-atexit
+CEXTRA = -fno-use-cxa-atexit -fno-threadsafe-statics
 
 CFLAGS := $(CDEBUG) $(CDEFS) $(CINCS) -O$(OPT) $(CWARN) $(CEXTRA) $(CTUNING) $(CSTANDARD)
 CXXFLAGS :=         $(CDEFS) $(CINCS) -O$(OPT) -Wall    $(CEXTRA) $(CTUNING) $(CXXSTANDARD)