lib: import a faster i386 memcpy() from uClibc

This takes between hundreds and thousands of clock cycles off the string
transfer benchmarks' median numbers. So it's quite fast indeed.

uClibc was licensed under the LGPL v2.1, original memcpy() function released
into the public domain by author.
test: string: multiple string items in full_string_buffers

By itself this isn't very useful at all. Needs scatter/gather, underdelivery,
and merging with various str_receiver_fn() users to eliminate overlap.