Fix stdin/stdout/stderr for pre-M.

This wasn't an array of pointers, it was an array of structs.
Unfortunately we need a complete type to index into the struct for
stdin/stdout/stderr, so add a phony struct that matches the size and
alignment of `struct __sFILE`. This property is guaranteed by the
static_asserts in libc/bionic/struct_file_test.cpp.

Test: mma
Bug: http://b/30465923
Change-Id: I8ce851dd64a261703bb44f9b5cd23b7caff4dd68