new gdb/gdba code has bug with absolute source filenames

[Bruce Stephens]

With a file (whose contents don't matter much) hello.c in /tmp.

     gcc -g3 -o hello hello.c

Then I can debug hello fine with the new M-x gdb.

If I compile it thus:

     gcc -g3 -o hello /tmp/hello.c

then when I do M-x gdb, and select gdb --annotate=3 hello, I see the
assembler code for main rather than C source code.

Obviously this is only a minor bug---overall the new modes are very
nice, even compared to the very usable previous version.