arm64: head.S: use memset to clear BSS

Currently we use an open-coded memzero to clear the BSS. As it is a
trivial implementation, it is sub-optimal.

Our optimised memset doesn't use the stack, is position-independent, and
for the memzero case can use of DC ZVA to clear large blocks
efficiently. In __mmap_switched the MMU is on and there are no live
caller-saved registers, so we can safely call an uninstrumented memset.

This patch changes __mmap_switched to use memset when clearing the BSS.
We use the __pi_memset alias so as to avoid any instrumentation in all
kernel configurations.

Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Marc Zyngier <marc.zyngier@arm.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>