x86_64 port and BSP (GSoC 2018)

[Amaan Cheval]

Quick update: My x86_64 stub port compiles and can be linked to all default
tests now! _dance_

I've pushed the stub port for the x86_64 to my fork on Github; the commits
are horribly messy (I did most of the work aiming for the stub to be one
commit, then later I realized several commits would make more sense for
when I'll have to rebase and catch up with master - TL;DR: ignore the
commits, I'll squash and make them make more sense later).

https://github.com/AmaanC/rtems/tree/ac/stub-x86-link-tests-pre-bspreorg-bak

It depends on the x86_64 tools being updated, so we need the patch I've
already made[1] to rtems-source-builder, along with one I'll make after my
patch to gcc[2] adding crti.o and crtn.o is accepted.

Using this configure command:

../kernel/configure --prefix=$HOME/bin/rtems/5-x86_64
--target=x86_64-rtems5 --enable-rtemsbsp=no_bsp --enable-posix
--enable-tests --enable-maintainer-mode

No errors were logged while running "make", and there are now 568 "*.exe"s
in the ./x86_64-rtems5/c/no_bsp/testsuites folders (using "find").

Next up:

- Restructure to use bsps dir
- Figure out specifics of linkcmds and bsp_specs (currently it's just a
hacky mashup of the one in no_bsp and pc386)
- Work out how

[1] https://lists.rtems.org/pipermail/devel/2018-April/020883.html
[2] https://gcc.gnu.org/ml/gcc-patches/2018-05/msg00007.html