arm64: Remove CONFIG_VMAP_STACK conditionals from THREAD_SHIFT and THREAD_ALIGN

arm64: Remove CONFIG_VMAP_STACK conditionals from THREAD_SHIFT and THREAD_ALIGN

Now that VMAP_STACK is always enabled on arm64, remove the
CONFIG_VMAP_STACK conditional logic from the definitions of THREAD_SHIFT
and THREAD_ALIGN in arch/arm64/include/asm/memory.h. This simplifies the
code by unconditionally setting THREAD_ALIGN to (2 * THREAD_SIZE) and
adjusting the THREAD_SHIFT definition to only depend on MIN_THREAD_SHIFT
and PAGE_SHIFT.

This change reflects the updated arm64 stack model, where all kernel
threads use virtually mapped stacks with guard pages, and ensures
alignment and stack sizing are consistently handled.

Signed-off-by: Breno Leitao <leitao@debian.org>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Link: https://lore.kernel.org/r/20250707-arm64_vmap-v1-3-8de98ca0f91c@debian.org
Signed-off-by: Will Deacon <will@kernel.org>