tools/nolibc/string: split the string functions into string.h

The string manipulation functions (mem*, str*) are now found in
string.h. The file depends on almost nothing and will be
usable from other includes if needed. Maybe more functions could
be added.

Signed-off-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>