libasm: Fix xdefault_pattern initialization

libasm: Fix xdefault_pattern initialization

clang complains:

    asm_newscn.c:48:22: error: field 'pattern' with variable sized type 'struct FillPattern' not at the end of a struct or class is a GNU extension [-Werror,-Wgnu-variable-sized-type-not-at-end]
      struct FillPattern pattern;
                         ^

Fix by using a union instead. Define the second union member to be a
char array 1 byte larger than struct FillPattern. This should be legal
according to 6.7.9:

    If an object that has static or thread storage duration is not
    initialized explicitly, then ... if it is a union, the first named
    member is initialized (recursively) according to these rules, and
    any padding is initialized to zero bits.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>