Blackfin: move string functions to normal lib/ assembly

Since 'extern inline' doesn't work correctly in the context of the Linux
kernel (too many overriding defines), move the string functions to normal
lib/ assembly files (like the existing mem funcs).  This avoids the forced
inline all over the kernel and allows us to place them constantly in L1.

This also avoids some module failures when gcc inserts calls to string
functions but the kernel build system doesn't fully consult the library
archives.

Signed-off-by: Robin Getz <robin.getz@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>