testsuite, fortran: make mixed-lang-stack less compiler dependent

testsuite, fortran: make mixed-lang-stack less compiler dependent

In the gdb.fortran/mixed-lang-stack.exp test when somewhere deep in a
bunch of nested function calls we issue and test a 'info args' command
for the mixed_func_1b function (when in that function's frame).

The signature of the function looks like

  subroutine mixed_func_1b(a, b, c, d, e, g)
    use type_module
    implicit none

    integer :: a
    real(kind=4) :: b
    real(kind=8) :: c
    complex(kind=4) :: d
    character(len=*) :: e
    character(len=:), allocatable :: f
    TYPE(MyType) :: g

and usually one would expect arguments a, b, c, d, e, and g to be
emitted here.  However, due to some compiler dependent treatment of the
e array the actual output in the test (with gfortran/ifx) is

  (gdb) info args
  a = 1
  b = 2
  c = 3
  d = (4,5)
  e = 'abcdef'
  g = ( a = 1.5, b = 2.5 )
  _e = 6

where the compiler generated '_e' is emitted as the length of e.  While
ifort also generates an additional length argument, the naming (which is
up to the compilers here I think, I could not find anything in the
Fortran standard about this) is different and we see

  (gdb) info args
  a = 1
  b = 2
  c = 3
  d = (4,5)
  e = 'abcdef'
  g = ( a = 1.5, b = 2.5 )
  .tmp.E.len_V$4a = 6

To make both outputs pass the test, I kept the additional argument for now and
made the regex for the emitted name of the last variable match any
arbitrary name.

Approved-by: Tom Tromey <tom@tromey.com>