This patch consolidates all the pread/pread64 implementation for Linux
in only one (sysdeps/unix/sysv/linux/pread.c). It also removes the
syscall from the auto-generation using assembly macros.
For pread{64} offset argument placement the new SYSCALL_LL{64} macro
is used. For pread ports that do not define __NR_pread will use
__NR_pread64 and for pread64 ports that dot define __NR_pread64 will
use __NR_pread for the syscall.
Checked on x86_64, x32, i386, aarch64, and ppc64le.