Detect NEON on AArch64

The __ARM_NEON is the standard define for NEON instructions support
__ARM_NEON__ is only legacy, and specifically not defined in
AArch64 builds, which causes us not to detect NEON support there.

The NEON assembler files doesn't build with AArch64, so the NEON
drawhelper methods must be excluded for now.

Change-Id: Ie32f855bde94ee7efd8a8ddb7766c931778e729b
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>