Linaro string handling routines: Give a option to use the older method

The newer method is compatible with all devices
Use export USE_OLD_MEMCPY=true
to use the older method
For ARM deivces that are not ARMV7A the newer method will be used either
way.

Signed-off-by:  Paul Beeler <sparksco@gmail.com>
Fix assembler error using newer toolchains.
Fixes a ".size expression for pow does not evaluate to a constant" assembler error using linaro 4.7.x.