linux: make write syscall available

The jemalloc memory allocator likes to bypass the libc when calling write, so
it calls the syscall directly. Let's make write available through this interface
as well.

Reviewed-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Glauber Costa <glommer@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>