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
diff --git a/libc/include/unistd.h b/libc/include/unistd.h
index aa236bd..4534fb9 100644
--- a/libc/include/unistd.h
+++ b/libc/include/unistd.h
@@ -145,6 +145,7 @@
 extern int fsync(int);
 extern int fdatasync(int);
 extern int ftruncate(int, off_t);
+extern int ftruncate64(int, off64_t);
 
 extern int pause(void);
 extern unsigned int alarm(unsigned int);