Now that support for 32-bit x86 has been removed, we don't have to worry
about the top half of `off_t`/`u64` values being chopped off when we try
to pass them in registers. Therefore, we no longer need the workaround
of pointers to stack-allocated values to syscalls.
Note that this changes the system call ABI, so statically linked
programs will have to be re-linked.
Previously we tried to determine if `fd` refers to a non-regular file by
doing a stat() operation on the file.
This didn't work out very well since many File subclasses don't
actually implement stat() but instead fall back to failing with EBADF.
This patch fixes the issue by checking for regular files with
File::is_regular_file() instead.
