MFC: r328834

MFC: r328834

o Let rtld(1) set up psABI user trap handlers prior to executing the
  objects' init functions instead of doing the setup via a constructor
  in libc as the init functions may already depend on these handlers
  to be in place. This gets us rid of:
  - the undefined order in which libc constructors as __guard_setup()
    and jemalloc_constructor() are executed WRT __sparc_utrap_setup(),
  - the requirement to link libc last so __sparc_utrap_setup() gets
    called prior to constructors in other libraries (see r122883).
  For static binaries, crt1.o still sets up the user trap handlers.
o Move misplaced prototypes for MD functions in to the MD prototype
  section of rtld.h.
o Sprinkle nitems().

git-svn-id: svn://svn.freebsd.org/base/stable/10@331206 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f