Even the freestanding lm32 compiler has <string.h> (because gcc builds
newlib by default as a minimal fallback).  Since I want to get rid of
pp_strlen and such stuff, we'd better include <string.h>, win size_t
and use the standard definitions for standard functions.

When we port to systems with no <string.h> we can provide it inside
its own arch- directory. So no harm is done and the code is simplified.

Signed-off-by: Alessandro Rubini <rubini@gnudd.com>