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

doesn'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: 24271