Adds pw_PreStaticConstructorInit() to allow ARMv7-M targets that use
pw_boot to perform initialization before C++ static constructors are
run. Updates stm32f429i-disc1 to enable FPU before static constructors
so static constructors can safely perform FPU operations if necessary.
Change-Id: I66cfa82096719f59c42c84e4bafd0e1740002e09