Linux: Clean up preadv2, pwritev2 system call names

Linux: Clean up preadv2, pwritev2 system call names

With the built-in tables __NR_preadv2 and __NR_pwritev2 are always
defined.

The kernel has never defined __NR_preadv64v2 and __NR_pwritev64v2
and is unlikely to do so, given that the preadv2 and pwritev2 system
calls themselves are 64-bit.

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>