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.
add runtime check for recent kernel ptrace support
add runtime check for seccomp-bpf support