Properly sign extend the result of jrand48() and mrand48().

These functions are supposed to return a value between [_2^31, 2^31).
This doesn't seem to work on 64-bit systems, where we return a value
between [0, 3^32). Patch up the function to use proper casts to int32_t.
While there, fix some other style bugs.

MFC after: 2 weeks