arc: Fix gcc-4.8 compilation failure for arc.c

arc: Fix gcc-4.8 compilation failure for arc.c

Building an arc target:

$ configulre --target=arc-elf32                \
             --enable-targets=arc-linux-uclibc \
	     ...

On a system with gcc-4.8 (CentOS 7.x), fails with:
--------8<---------
../../gdb/arch/arc.c:117:43:   required from here
/usr/include/c++/4.8.2/bits/hashtable_policy.h:195:39: error: no matching
function for call to 'std::pair<const arc_arch_features, const
std::unique_ptr<target_desc, target_desc_deleter> >::pair(const
arc_arch_features&, target_desc*&)'
  : _M_v(std::forward<_Args>(__args)...) { }
                                       ^
/usr/include/c++/4.8.2/bits/hashtable_policy.h:195:39: note: candidates are:
In file included from /usr/include/c++/4.8.2/utility:70:0,
                 from /usr/include/c++/4.8.2/tuple:38,
                 from /usr/include/c++/4.8.2/functional:55,
                 from ../../gdb/../gdbsupport/ptid.h:35,
                 from ../../gdb/../gdbsupport/common-defs.h:123,
                 from ../../gdb/arch/arc.c:19:
/usr/include/c++/4.8.2/bits/stl_pair.h:206:9: note: template<class ...
_Args1, long unsigned int ..._Indexes1, class ... _Args2, long unsigned int
..._Indexes2> std::pair<_T1, _T2>::pair(std::tuple<_Args1 ...>&,
std::tuple<_Args2 ...>&, std::_Index_tuple<_Indexes1 ...>,
std::_Index_tuple<_Indexes2 ...>)
         pair(tuple<_Args1...>&, tuple<_Args2...>&,
         ^
-------->8---------

The corresponding line in arc.c must use an explicit ctor:
--------8<---------
 arc_lookup_target_description (...)
 {

   /* Add the newly created target description to the repertoire.  */
-  arc_tdesc_cache.emplace (features, tdesc);
+  arc_tdesc_cache.emplace (features, target_desc_up (tdesc));

   return tdesc;
 }
-------->8---------
See "PR gcc/96537" for more details.

Last but not least, this problem has originally been investigated
by Tom de Vries for RISCV targets (see 38f8aa06d9).

gdb/ChangeLog:

	PR build/27385
	* arch/arc.c (arc_lookup_target_description): Use
	target_desc_up() ctor explicitly.