Add x86_64 EFI start function from GNU-EFI project.

Original file is gnuefi/crt0-efi-x86_64.S  in GNU-EFI.

From GNU-EFI commit 8f5ba0e
http://sourceforge.net/p/gnu-efi/code/ci/8f5ba0e4fd398d49c0004164d18e50fb6acd36ca/
Rename to start_func.S

Use a generic filename for start function. The directory tells what arch
the function is for so we can use simpler logic in the Jamfile.
Working EFI application ('Hello World') and Jamfile.

First version of an EFI Application that actually runs.
The Jamfile is a still a bit of a hack, but builds with updated buildtools.

I currently build by adding
SubInclude HAIKU_TOP src system boot platform efi ;
to src/system/boot/Jamfile and running jam haiku.efi but that is just a
temporary hack.
There are still things to clean up in Jamfile, for instance haiku.efi is put
in the haiku directory atm. This is however first working version. To set up
QEMU to test haiku.efi see my docs in https://github.com/tqh/efi-example

Now it is 'just' the work of writing a bootloader left.