linux(4): Preserve %rcx (return address) like a Linux do

Perhaps, this does not makes much sense as destroyng %rcx declared by
the x86_64 Linux syscall ABI. However,:
a) if we get a signal while we are in the kernel, we should restore

tf_rcx when preparing machine context for signal handlers.

b) the Linux world is strange, someone can depend on %rcx value

after syscall, something like go.

Differential Revision: https://reviews.freebsd.org/D40155
MFC after: 1 month