rearrange x86-64 vdso symbol monkeypatching for clarity
define RR_HIDDEN in preload.c for convenience
fixup sizeof(sizeof(syscallbuf_hdr)) thinkos

This expression computes the desired result on 32-bit platforms, but
returns something quite different on 64-bit platforms, leading to replay
and dump failures when we actually have syscallbuf data in the trace.
fix lingering 32-bit-isms in the preload code

The fix to stored_record_size ensures that we detect the proper
condition in start_commit_buffered_syscall.  The fix to
commit_raw_syscall ensures that we don't incorrectly truncate syscall
return values.