ld/dwarf reports 'corrupt z stack' for source files with lots of line directives

<pre>.. such as are generated by goyacc.

fix  CL 4938042 / 5671737303a0 for <a href="https://golang.org/issue/1878">issue #1878</a> broke the attached testcase</pre>


Attachments:
1. <a href="https://storage.googleapis.com/go-attachment/2241/0/linedirectives.go">linedirectives.go</a> (1138 bytes)
