[wip] gdbserver, x86: support fs_base and gs_base

When debugging over gdbserver fs/gs_base is not updated when changing the
corresponding selector.

For 32-bit inferiors, fs/gs_base are now defined but not fetched.

Change-Id: I5a4d8637324ee5f0c303fdcc5eeb8ca50b96c577
Signed-off-by: Markus Metzger  <markus.t.metzger@intel.com>