Patch the vdso __kernel_vsyscall() to jump into librrpreload.

This allows rr to buffer libc syscalls that originate at non-POSIX (or
implementation-specific, backwards-compatibility) symbols.  Most
important among these is SYS_futex calls, which are the vast majority
of unbuffered FF syscalls.

Implementations of buffered syscalls in the new scheme will be added
one-by-one as their correctness is established.