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.