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.
use locked state of syscallbuf for determining in-syscallbuf-ness
turn on syscall buffering for x86-64 vdso syscalls
add support for finding where lib{c,pthread} live
factor out finding elf symbols
add support for generating field offsets from assembly templates
add Marker functionality to assembly templates
turn on syscall buffering for non-cancellation points in libc
turn on syscall buffering for cancellation points in libc