Fix all warnings for native and qemu-i386

Missing returns, unused variables (only used for debugging), empty
translation units, missing function prototypes, and GNU extensions.
According to http://stackoverflow.com/questions/2280492/bit-fields-of-type-other-than-int:

> 6.7.2.1 Structure and union specifiers

>  A bit-field shall have a type that is a qualified or unqualified version of _Bool, signed int, unsigned int, or some other implementation-defined type.