libc: Add ftruncate64 and improve 64-bit parameter syscall handling.

This patch improves the handling of 64-bit parameters in syscalls on ARM.
The ARM EABI mandates that 64-bit quantities be passed in even/odd register
pairs, which requires special treatment.

This allows us to simplify our implementations of pread() and pwrite()
and remove the C stubs for pread64() and pwrite64().

Also add ftruncate64() to <unistd.h>

Change-Id: I407e2fd223ba0093dd2d0b04c6152fadfc9ce3ef

Bug 3107933

libc/unistd/pwrite.c

@@ -28,10 +28,8 @@
 #include <sys/types.h>
 #include <unistd.h>
 
-extern int __pwrite64(int fd, const void *buf, size_t nbytes, loff_t offset);
-
 ssize_t pwrite(int fd, const void *buf, size_t nbytes, off_t offset)
 {
-    return __pwrite64(fd, buf, nbytes, offset);
+    return pwrite64(fd, buf, nbytes, (off64_t)offset);
 }