posix/regcomp: initialize union structure tag to avoid warning

We see some surprising warnings on tilegx with gcc 4.8.2:

In file included from regex.c:66:0:
regcomp.c: In function ‘parse_expression’:
regcomp.c:2849:15: error: ‘end_elem’ may be used uninitialized in this
function [-Werror=maybe-uninitialized]
       else if (br_elem->type == COLL_SYM)
               ^
regcomp.c:3109:34: note: ‘end_elem’ was declared here
       bracket_elem_t start_elem, end_elem;
                                  ^
regcomp.c:3109:22: error: ‘start_elem’ may be used uninitialized in
this function [-Werror=maybe-uninitialized]
       bracket_elem_t start_elem, end_elem;
                      ^

These warnings are not seen on x86, and in fact if I compile the
preprocessed tile sources with the x86 gcc 4.8.2, I don't see the
warnings.  I do see eqiuvalent warnings if I compile the
x86-preprocessed source code with tilegx gcc 4.8.2.

The fix here is to initialize the union type field appropriately in
a couple of places where we pass a union pointer to a subroutine that
"knows" what type the union is.  Setting the type explicitly seems like
a more robust way to manage such a data structure in any case.