bionic: add support for non-NEON memcpy() on NEON SoCs

Some SoCs that support NEON nevertheless perform better with a non-NEON than a
NEON memcpy().  This patch adds build variable ARCH_ARM_USE_NON_NEON_MEMCPY,
which can be set in BoardConfig.mk.  When ARCH_ARM_USE_NON_NEON_MEMCPY is
defined, we compile in the non-NEON optimized memcpy() even if the SoC supports
NEON.

Change-Id: Ia0e5bee6bad5880ffc5ff8f34a1382d567546cf9