csu: Add the prologue and epilogue to the _init and _fini on x86_64

Normally, modern unwinders uses Dwarf information to unwind stack,
however in case when the code is not annotated by Dwarf instructions,
unwinders fallbacks to a frame-pointer based algorithm.

That is allows libunwind to unwind stack from global constructors and
destructors. Also it makes gdb happy as it printed nonexistent frame
before.

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D40795