CTF: handle forward reference type

The problems can be illustrated, with any program, below:

(gdb) print main
$1 = {main} 0x0

The return type was incorrectly set in read_func_kind_type, with
the name of the function, which leads c_type_print_base_1 to print
it. In addition, the address of a new function needs to be set with
that info in its minimal symtab entry, when the new function is added.

After the fix:

(gdb) print main
$1 = {int ()} 0x4004b7 <main>

A new test, gdb.ctf/funcreturn.exp, is added to the testsuite.

gdb/ChangeLog:
        * ctfread.c (new_symbol): Set function address.
        (read_func_kind_type): Remove incorrect type name setting.
        Don't copy name returned from ctf_type_ame_raw throughout file.

gdb/testsuite/ChangeLog:
        * gdb.ctf/funcreturn.exp: New file.
        * gdb.ctf/whatis.c: Copy from gdb.base.