scripts/gdb: set vabits_actual based on TCR_EL1

scripts/gdb: set vabits_actual based on TCR_EL1

We encounter the following issue after commit 9cce9c6c2c3b ("arm64: mm: Handle
LVA support as a CPU feature").

(gdb) lx-slabinfo
Python Exception <class 'gdb.error'>: No symbol "vabits_actual" in current context.
Error occurred in Python: No symbol "vabits_actual" in current context.

We set vabits_actual based on TCR_EL1 value when
VA_BITS is bigger than 48.

Link: https://lkml.kernel.org/r/20240619074911.100434-5-kuan-ying.lee@canonical.com
Fixes: 9cce9c6c2c3b ("arm64: mm: Handle LVA support as a CPU feature")
Signed-off-by: Kuan-Ying Lee <kuan-ying.lee@canonical.com>
Cc: Jan Kiszka <jan.kiszka@siemens.com>
Cc: Kieran Bingham <kbingham@kernel.org>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>