default-initialization of bitfields

default-initialization of bitfields

The code:

struct bf_SS {unsigned int bit:1,bits31:31; };
void func(void) {
  struct bf_SS bf_finit = { .bit = 1 };
}

will not init bits31 to 0.

tccgen.c:
- check_bf: New function to check if bitfield is present in struct/union
- decl_initializer: Call check_bf and set value to 0 is bitfield found

tests/tcctest.c:
- Add struct bitfield test code