x86-64: Add p{read,write}[v]64 to syscalls.list [BZ #20348]

64-bit off_t in pread64, preadv, pwrite64 and pwritev syscalls is passed
in one 64-bit register for both x32 and x86-64.  Since the inline
asm statement only passes long, which is 32-bit for x32, in registers,
64-bit off_t is truncated to 32-bit on x32.  Since __ASSUME_PREADV and
__ASSUME_PWRITEV are defined unconditionally, these syscalls can be
implemented in syscalls.list to pass 64-bit off_t in one 64-bit register.

Tested on x86-64 and x32 with off_t > 4GB on pread64/pwrite64 and
preadv64/pwritev64.

	[BZ #20348]
	* sysdeps/unix/sysv/linux/x86_64/syscalls.list: Add pread64,
	preadv64, pwrite64 and pwritev64.