linux-user: Merge pread/pwrite into pread64/pwrite64

linux-user: Merge pread/pwrite into pread64/pwrite64

The Linux syscalls underlying pread() and pwrite() take a 64 bit
offset on all architectures, even if some of them name the syscall
"pread/pwrite" rather than "pread64/pwrite64" for historical reasons.
So move the four QEMU target architectures (arm, i386, sparc,
unicore32) which were defining TARGET_NR_pread/pwrite to define
TARGET_NR_pread64/pwrite64 instead, and drop the TARGET_NR_pread/pwrite
implementation code completely.

(Based on examination of the kernel sources for the four architectures
this patch affects.)

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>