- Map the __builtin_nan(f|l)? functions to nan(f|l)? on msvc
- MSVC throws C2099 when initializing a struct with cast syntax
in a global initializer.  Added zig_as_init_  to handle this case,
 and generate it only in static initializers for > 64 bit ints.
- Change float initialization to emit the integer representation
in global initializers to avoid C2099 caused by calling nan.