std: Prefer 64bit libc functions where possible

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.