lib: libc: arcmwdt: move ssize_t definition from the toolchain header

Until POSIX layer decoupling is done, we still need the size_t type
definition across the codebase, but it should come through sys/types.h
and not through toolchain-specific headers.

Signed-off-by: Ilya Tagunov <Ilya.Tagunov@synopsys.com>