add support for .asciz, and enable it by default. If your target...

add support for .asciz, and enable it by default.  If your target assemblerdoesn't support .asciz, just set AscizDirective to null in your asmprinter.
This compiles C strings to:

l1__2E_str_1:                           ; '.str_1'
        .asciz  "foo"

instead of:

l1__2E_str_1:                           ; '.str_1'
        .ascii  "foo\000"

llvm-svn: 24272