gdb: remove two uses of obstack

Remove uses of obstack in the code generating the libraries XML for
Windows and AIX.  Use std::string instead.  I'm not able to test this
change, unfortunately.

Change-Id: I28480913337e3fe8d6c31e551626931e6b1367ef
Approved-By: Tom Tromey <tom@tromey.com>
4 files changed