[gdb/testsuite] Fix fail in gdb.fortran/ptype-on-functions.exp with gcc-7

Since commit 05b85772 "gdb/fortran: Add type info of formal parameter for
clang" I see:
...
(gdb) ptype say_string^M
type = void (character*(*), integer(kind=4))^M
(gdb) FAIL: gdb.fortran/ptype-on-functions.exp: ptype say_string
...

The part of the commit causing the fail is:
...
 gdb_test "ptype say_string" \
-    "type = void \\(character\\*\\(\\*\\), integer\\(kind=\\d+\\)\\)"
+    "type = void \\(character\[^,\]+, $integer8\\)"
...
which fails to take into account that for gcc-7 and before, the type for
string length of a string argument is int, not size_t.

Fix this by allowing both $integer8 and $integer4.

Tested on x86_64-linux, with gcc-7 and gcc-10.

gdb/testsuite/ChangeLog:

2021-07-05  Tom de Vries  <tdevries@suse.de>

	* gdb.fortran/ptype-on-functions.exp: Allow both $integer8 and
	$integer4 for size of string length.