[gdb/testsuite] Fix gdb.arch/arm-single-step-kernel-helper.exp

[gdb/testsuite] Fix gdb.arch/arm-single-step-kernel-helper.exp

On arm-linux I run into:
...
(gdb) p *kernel_user_helper_version^M
Cannot access memory at address 0xffff0ffc^M
(gdb) FAIL: gdb.arch/arm-single-step-kernel-helper.exp: check kernel helper version
...

What the test-case is trying to do, is to access a special address in the arm
linux kernel [1] using ptrace, which doesn't seem to work.

This is with kernel version 6.1.55.  Perhaps this used to work, but the kernel
was modified to be more strict with respect to access to this special address.

Fix this by making the inferior access that special address instead.

Tested on arm-linux.

Approved-By: Luis Machado <luis.machado@arm.com>
PR testsuite/32070
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=32070

[1] https://www.kernel.org/doc/Documentation/arm/kernel_user_helpers.txt