microblaze: Guard __HAVE_ARCH macros with __KERNEL__ in string.h

A polarity reversal in the __KERNEL__ guard prevents the __HAVE_ARCH
flags from being defined in kernel compilation.

I noticed that there's now an option for assembly-optimized versions of
memcpy and memmove. I believe this may be buggy; when I turn it on, all
my printk output gets smashed together, as if the newlines aren't getting
copied.

Signed-off-by: Steven J. Magnani <steve@digidescorp.com>
Signed-off-by: Michal Simek <monstr@monstr.eu>