split vsyscall_hook_trampoline out to a separate file

Fewer toplevel asms are a good thing.
remove get_vsyscall_hook_trampoline

Now that we have real declarations for _vsyscall_hook_trampoline visible
to the C compiler, we can delete the get_vsyscall_hook_trampoline
function and let the compiler do all the work of figuring out where
_vsyscall_hook_trampoline lives.
make vsyscall_hook_trampoline do a direct call to vsycall_hook

This is a stopgap fix so that we can turn on PIC for the preloaded
library.  Really, everything in this library should probably be hidden,
with the initialization code setting everything up.