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/docs/CHANGES.TXT b/libc/docs/CHANGES.TXT
index 8e23c0e..3fbbee0 100644
--- a/libc/docs/CHANGES.TXT
+++ b/libc/docs/CHANGES.TXT
@@ -14,6 +14,8 @@
 - <sched.h>: Add sched_getcpu(), sched_getaffinity(), sched_setaffinity(),
   cpu_set_t and related macros (e.g. CPU_SETSIZE, CPU_ZERO, CPU_SET, ...)
 
+- <unistd.h>: Add ftruncate64().
+
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 Differences between Android 2.3 and Android 2.2: