Fix (and extend) src/test/run-pass/foreign-call-no-runtime.rs

Fix (and extend) src/test/run-pass/foreign-call-no-runtime.rs

While going over various problems signaled by valgrind when running
`make check` on a build configured with `--enable-valgrind`, I
discovered a bug in this test case.

Namely, the test case was previously creating an `i32` (originally an
`int` aka `isize` but then we changed the name and the fallback
rules), and then reading from a `*const isize`. Valgrind rightly
complains about this, since we are reading an 8 byte value on 64-bit
systems, but in principle only 4 bytes have been initialized.

(I wish this was the only valgrind unclean test, but unfortunately
there are a bunch more. This was just the easiest/first one that I
dissected.)