Block signals during the initial part of dlopen

Lazy binding in a signal handler that interrupts a dlopen sees
intermediate dynamic linker state.  This has likely been always
unsafe, but with the new pending NODELETE state, this is clearly
incorrect.  Other threads are excluded via the loader lock, but the
current thread is not.  Blocking signals until right before ELF
constructors run is the safe thing to do.

Change-Id: Iad079080ebe7442c13313ba11dc2797953faef35