linux-user: Build vdso for x64.

... Well, sortof.  The Makefile bits are broken.
Patch to load the vdso into the running program to follow.

Signed-off-by: Richard Henderson <rth@twiddle.net>
linux-user: Load a VDSO for x86-64.

Signed-off-by: Richard Henderson <rth@twiddle.net>