Fix bitfields with non-int types and in unions

Fix bitfields with non-int types and in unions

The ISO C draft allow only signed/unsigned int and _Bool as base type
for bitfields. TinyCC ever since allowed a wider range of types, but
there were many bugs that shifted values when they shouldn't, etc..
The patch introduces a restriction to the layout of bitfields with
mixed types that makes it incompatible with GCC. In

struct {
  typeA x:1;
  typeB y:1;
};

y is combined with x in the same byte iff typeA is typeB (neglecting
signedness). This is done to avoid alignment issues and exceeding the
width of typeA.