gdb: Fix abort w/NIX_DEBUG_INFO_DIRS patch (#80929)

The current gdb patch to support NIX_DEBUG_INFO_DIRS fails if
the user attempts to change the debug file directory to a
value other than the default. For instance:

(gdb) set debug-file-directory /run/booted-system/sw/lib/debug

results in:

munmap_chunk(): invalid pointer
Aborted (core dumped)

To fix this issue, the debug_file_directory is allocated
with xstrdup so that a subsequent call to xfree will succeed.