Fix build with gcc-4.8.x

Use an explicit conversion from unique_ptr<T> to
displaced_step_closure_up to avoid a compiler bug
with gcc-4.8.4:

../../binutils-gdb/gdb/amd64-tdep.c:1514:10: error: cannot bind
   'std::unique_ptr<amd64_displaced_step_closure>' lvalue to
   'std::unique_ptr<amd64_displaced_step_closure>&&'

gdb:
2020-02-18  Bernd Edlinger  <bernd.edlinger@hotmail.de>

	* aarch64-tdep.c (aarch64_displaced_step_copy_insn): Use an explicit
	conversion.
	* amd64-tdep.c (amd64_displaced_step_copy_insn): Likewise.
	* arm-linux-tdep.c (arm_linux_displaced_step_copy_insn): Likewise.
	* i386-tdep.c (i386_displaced_step_copy_insn): Likewise.
	* rs6000-tdep.c (ppc_displaced_step_copy_insn): Likewise.
	* s390-tdep.c (s390_displaced_step_copy_insn): Likewise.