[gdb/build] Fix missing implicit constructor call with gcc 4.8

When building gdb on x86_64-linux with --enable-targets riscv64-suse-linux, I
run into:
...
src/gdb/arch/riscv.c:112:45:   required from here
/usr/include/c++/4.8/bits/hashtable_policy.h:195:39: error: no matching \
  function for call to 'std::pair<const riscv_gdbarch_features, const \
  std::unique_ptr<target_desc, target_desc_deleter> >::pair(const \
  riscv_gdbarch_features&, target_desc*&)'
  : _M_v(std::forward<_Args>(__args)...) { }
                                       ^
...
for this code in riscv_lookup_target_description:
...
  /* Add to the cache.  */
  riscv_tdesc_cache.emplace (features, tdesc);
...

Work around this compiler problem (filed as PR gcc/96537), similar to how that
was done in commit 6d0cf4464e "Fix build with gcc-4.8.x":
...
-  riscv_tdesc_cache.emplace (features, tdesc);
+  riscv_tdesc_cache.emplace (features, target_desc_up (tdesc));
...
That is, call the target_desc_up constructor explictly instead of implicitly.

Also, work around a similar issue in get_thread_arch_aspace_regcache.

Build on x86_64-linux with --enable-targets riscv64-suse-linux, and
reg-tested.

gdb/ChangeLog:

2020-08-08  Tom de Vries  <tdevries@suse.de>

	PR build/26344
	* arch/riscv.c (riscv_lookup_target_description): Use an explicit
	constructor.
	* regcache.c (get_thread_arch_aspace_regcache): Same.

gdb/ChangeLog

@@ -1,3 +1,10 @@
+2020-08-08  Tom de Vries  <tdevries@suse.de>
+
+	PR build/26344
+	* arch/riscv.c (riscv_lookup_target_description): Use an explicit
+	constructor.
+	* regcache.c (get_thread_arch_aspace_regcache): Same.
+
 2020-08-07  Tom Tromey  <tromey@adacore.com>
 
 	* ravenscar-thread.c

gdb/arch/riscv.c

@@ -108,8 +108,9 @@ riscv_lookup_target_description (const struct riscv_gdbarch_features features)
 
   target_desc *tdesc = riscv_create_target_description (features);
 
-  /* Add to the cache.  */
-  riscv_tdesc_cache.emplace (features, tdesc);
+  /* Add to the cache.  Work around a problem with g++ 4.8 (PR96537):
+     Call the target_desc_up constructor explictly instead of implicitly.  */
+  riscv_tdesc_cache.emplace (features, target_desc_up (tdesc));
 
   return tdesc;
 }

gdb/regcache.c

@@ -354,7 +354,9 @@ get_thread_arch_aspace_regcache (process_stratum_target *target,
   /* It does not exist, create it.  */
   regcache *new_regcache = new regcache (target, arch, aspace);
   new_regcache->set_ptid (ptid);
-  ptid_regc_map.insert (std::make_pair (ptid, new_regcache));
+  /* Work around a problem with g++ 4.8 (PR96537): Call the regcache_up
+     constructor explictly instead of implicitly.  */
+  ptid_regc_map.insert (std::make_pair (ptid, regcache_up (new_regcache)));
 
   return new_regcache;
 }