arm64: remove CONFIG_VMAP_STACK checks from SDEI stack handling

arm64: remove CONFIG_VMAP_STACK checks from SDEI stack handling

With VMAP_STACK now always enabled on arm64, remove all
CONFIG_VMAP_STACK conditionals from SDEI stack allocation and
initialization in arch/arm64/kernel/sdei.c.

This change unconditionally defines the SDEI stack pointers and replaces
runtime checks with BUILD_BUG_ON() assertions, ensuring that the code is
only built when VMAP_STACK is enabled. This simplifies the logic and
reflects the mandatory use of VMAP_STACK for all arm64 kernel builds.

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-v1-7-8de98ca0f91c@debian.org
Signed-off-by: Will Deacon <will@kernel.org>