gdbserver/linux-x86: move lwp declaration out of __x86_64__ region

gdbserver/linux-x86: move lwp declaration out of __x86_64__ region

Commit 4855cbdc3d8f ("gdbserver/linux-x86: make is_64bit_tdesc accept
thread as a parameter") caused this when building in 32 bits / i386
mode:

      CXX    linux-x86-low.o
    In file included from /home/smarchi/src/binutils-gdb/gdbserver/linux-x86-low.cc:24:
    /home/smarchi/src/binutils-gdb/gdbserver/linux-x86-low.cc: In member function ‘virtual int x86_target::low_get_thread_area(int, CORE_ADDR*)’:
    /home/smarchi/src/binutils-gdb/gdbserver/linux-x86-low.cc:357:47: error: ‘lwp’ was not declared in this scope
      357 |     struct thread_info *thr = get_lwp_thread (lwp);
          |                                               ^~~
    /home/smarchi/src/binutils-gdb/gdbserver/linux-low.h:709:31: note: in definition of macro ‘get_lwp_thread’
      709 | #define get_lwp_thread(lwp) ((lwp)->thread)
          |                               ^~~

This is because it moved the lwp variable declaration inside the
__x86_64__ guard, making it unavailable when building in 32 bits mode.
Move the lwp variable outside of the __x86_64__ region.

Change-Id: I7fa3938c6b44b345c27a52c8b8d3ea12aba53e05