linux-ppc-low.c: Remove forward declaration, move ppc_arch_setup lower

linux-ppc-low.c: Remove forward declaration, move ppc_arch_setup lower

g++ doesn't like that we forward-declare a variable that is initialized
later in the file.  It's easy enough to re-order things to fix it.

Fixes

/home/simark/src/binutils-gdb/gdb/gdbserver/linux-ppc-low.c:663:28: error: redefinition of ‘usrregs_info ppc_usrregs_info’
 static struct usrregs_info ppc_usrregs_info =
                            ^
/home/simark/src/binutils-gdb/gdb/gdbserver/linux-ppc-low.c:381:28: note: ‘usrregs_info ppc_usrregs_info’ previously declared here
 static struct usrregs_info ppc_usrregs_info;
                            ^

gdb/gdbserver/ChangeLog:

	* linux-ppc-low.c (ppc_usrregs_info): Remove
	forward-declaration.
	(ppc_arch_setup): Move lower in file.