x86 LynxOS-178: Adjust floating-point context structure

x86 LynxOS-178: Adjust floating-point context structure

The floating point context structure on x86 LynxOS-178 is not
the same as on LynxOS 5.x. As a consequence, trying to print
the return value of a function returning a float, for instance,
yields incorrect results.

This patch fixes the issue by providing an updated definition
for LynxOS-178 (the reason why we cannot access the actual definition
provided by the system still remains true).

gdb/gdbserver/ChangeLog:

        * lynx-i386-low.c (LYNXOS_178): New macro.
        [LYNXOS_178] (usr_fcontext_t): Provide a definition that matches
        the layout on LynxOS-178.
        (lynx_i386_fill_fpregset, lynx_i386_store_fpregset): Do not
        handle floating point registers that are not supported by
        LynxOS-178.