arm64: mm: remove unnecessary EXPORT_SYMBOL_GPL

arch_pick_mmap_layout is only called by fs/exec.c which is always built into
kernel, it looks the EXPORT_SYMBOL_GPL is pointless and no architectures export
it other than ARM64.

Signed-off-by: Yang Shi <yang.shi@linaro.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>