Handle PPC64 function descriptor in Ada decoding

On PPC64, the entry point of the function "FN" is ".FN" when a function
descriptor is used. One of the consequences of this is that GDB then
presents the name of the function to the user (eg: in backtraces) with
the leading dot, which is a low-level internal detail that the user
should not be seeing.  The Ada decoding should strip it.

gdb/ChangeLog:

	* ada-lang.c (ada_decode): strip dot prefix in symbol name.

No testcase added, as a number of existing testcases should already
demonstrate that problem.

gdb/ChangeLog

@@ -1,3 +1,7 @@
+2018-09-08  Jerome Guitton  <guitton@adacore.com>
+
+	* ada-lang.c (ada_decode): strip dot prefix in symbol name.
+
 2018-09-08  Joel Brobecker  <brobecker@adacore.com>
 
 	* ada-lang.c (ada_exception_sal): Replace gdb_assert calls

gdb/ada-lang.c

@@ -1164,6 +1164,11 @@ ada_decode (const char *encoded)
   static char *decoding_buffer = NULL;
   static size_t decoding_buffer_size = 0;
 
+  /* With function descriptors on PPC64, the value of a symbol named
+     ".FN", if it exists, is the entry point of the function "FN".  */
+  if (encoded[0] == '.')
+    encoded += 1;
+
   /* The name of the Ada main procedure starts with "_ada_".
      This prefix is not part of the decoded name, so skip this part
      if we see this prefix.  */