gdb: fix possible uninitialised variable use

After this commit:

  commit 198ff6ff
  Date:   Tue Jan 30 15:37:23 2024 +0000

      gdb/gdbserver: share x86/linux tdesc caching

a possible use of an uninitialised variable was introduced, the
'tdesc' variable in i386_linux_core_read_description might be read
without being written too if 'xcr0' was 0.

This is fixed in this commit.  I've updated the function to follow the
same pattern as amd64_linux_core_read_description, if xcr0 is 0 then
we select a default xcr0 value and use that to select a tdesc.