[gdb/testsuite] Fix gdb.fortran/array-slices.exp with -m32

[gdb/testsuite] Fix gdb.fortran/array-slices.exp with -m32

When running test-case gdb.fortran/array-slices.exp with target board
unix/-m32, we run into:
...
(gdb) print /x &array4d^M
$69 = 0xffffb620^M
(gdb) print /x (&array4d) + sizeof (array4d)^M
$70 = 0x95c620^M
(gdb) FAIL: gdb.fortran/array-slices.exp: repack=on: test 9: check sizes match
...
The expressions calculate the start and end of an array, but the calculation
of the end expression has an unexpected result (given that it lies before the
start of the array).  By printing "sizeof (array4d)" as a separate
expression:
...
(gdb) print /x sizeof (array4d)
$1 = 0xc40
...
it becomes clear we expected to get 0xffffb620 + 0xc40 == 0xffffc260 instead.

The problem is that using the '&' returns a pointer type:
...
(gdb) p &array4d
$5 = (PTR TO -> ( integer(kind=4) (-3:3,7:10,-3:3,-10:-7) )) 0xffffbe00
...
which has the consequence that the addition is done as pointer arithmetic.

Fix this by using the result of "print /x &array4d" instead of &array4d in the
addition.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2021-01-15  Tom de Vries  <tdevries@suse.de>

	PR testsuite/26997
	* gdb.fortran/array-slices.exp (run_test): Avoid pointer arithmetic
	when adding sizeof.