gdb/testsuite: Update test pattern in ptype-on-functions.exp

gdb/testsuite: Update test pattern in ptype-on-functions.exp

It was pointed out that the recently added test
gdb.fortran/ptype-on-functions.exp fails on older versions of
gfortran.  This is because the ABI for passing string lengths changed
from a 4-byte to 8-byte value (on some targets).

This change is documented here:
  https://gcc.gnu.org/gcc-8/changes.html.

  Character variables longer than HUGE(0) elements are now possible on
  64-bit targets. Note that this changes the procedure call ABI for
  all procedures with character arguments on 64-bit targets, as the
  type of the hidden character length argument has changed. The hidden
  character length argument is now of type INTEGER(C_SIZE_T).

This commit just relaxes the pattern to accept any size of integer for
the string length argument.

gdb/testsuite/ChangeLog:

	* gdb.fortran/ptype-on-functions.exp: Make the result pattern more
	generic.