Fix configure.srv error for Linux on PowerPC

An error in commit 42cd72aa caused
srv_tgtobj to be overwritten and linux-ppc-low.o to be missed when
linking gdbserver for Linux on PowerPC.  This patch fixes the error.

gdb/gdbserver/ChangeLog:
2020-01-29  Pedro Franco de Carvalho  <pedromfc@linux.ibm.com>

	* configure.srv (powerpc*-*-linux*): Use srv_tgtobj in second
	assignment instead of srv_linux_obj.