gdb: implement linux namespace support for fileio_lstat and vFile::lstat

gdb: implement linux namespace support for fileio_lstat and vFile::lstat

The new algorithm to look for a build-id-based debug file
(introduced by commit 22836ca88591ac7efacf06d5b6db191763fd8aba)
makes use of fileio_lstat. As lstat was not supported by
linux-namespace.c, all lstat calls would be performed on the host
and not inside the namespace.  Fixed by adding namespace lstat
support.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=32956

Approved-By: Andrew Burgess <aburgess@redhat.com>