gdb: parse and set the inferior environment from core files

Extend the core file context parsing mechanism added in the previous
commit to also store the environment parsed from the core file.

This environment can then be injected into the inferior object.

The benefit of this is that when examining a core file in GDB, the
'show environment' command will now show the environment extracted
from a core file.

Consider this example:

  $ env -i GDB_TEST_VAR=FOO ./gen-core
  Segmentation fault (core dumped)
  $ gdb -c ./core.1669829
  ...
  [New LWP 1669829]
  Core was generated by `./gen-core'.
  Program terminated with signal SIGSEGV, Segmentation fault.
  #0  0x0000000000401111 in ?? ()
  (gdb) show environment
  GDB_TEST_VAR=foo
  (gdb)

There's a new test for this functionality.

gdb/arch-utils.c

@@ -1499,6 +1499,32 @@ gdbarch_initialized_p (gdbarch *arch)
   return arch->initialized_p;
 }
 
+/* See arch-utils.h.  */
+
+gdb_environ
+core_file_exec_context::environment () const
+{
+  gdb_environ e;
+
+  for (const auto &entry : m_environment)
+    {
+      char *eq = strchr (entry.get (), '=');
+
+      /* If there's no '=' character, then skip this entry.  */
+      if (eq == nullptr)
+	continue;
+
+      const char *value = eq + 1;
+      const char *var = entry.get ();
+
+      *eq = '\0';
+      e.set (var, value);
+      *eq = '=';
+    }
+
+  return e;
+}
+
 void _initialize_gdbarch_utils ();
 void
 _initialize_gdbarch_utils ()