While musl decided to hard-wire off_t to a 64bit quantity, glibc is much
older and defaults to 32bit offsets and offers some -64 suffixed
versions of I/O functions.

There's a weird mix-up of types: sometimes off_t is used, sometimes not,
sometimes it's defined as a signed quantity and sometimes as an unsigned
one, but we'll sort this problem later.

They return c_int and not isize.

This is needed to catch any possible problem with executables linking to
32bit glibc.

Let's follow the libc/kernel convention and use an i64 for offsets, we
bitcast as needed and avoid the useless conditional casts.

It turns out that nothing in the test suite was exercising
preadv/pwritev and so the previous commits silently broke them.

Adding tests revealed readvAll and preadvAll were also broken and not
covered by any test.

Use the same order as Clang (and, by extension, GCC) for the three most
important libc components: lm comes first, followed by lpthread and then
lc.