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>