Resuming old EFI project

[Eric McCorkle]

Hello everyone,

About two years ago, I was working on a project to try and finish the 
EFI support for AMD64.  Unfortunately, I had to abandon the project due 
to family illness and a job search.  At this point, I'm in a position to 
pick that project back up as a side project (I also have better hardware 
now ;)

It seems everything in sys/boot/amd64/efi is pretty much unchanged.  One 
thing that's changed things quite a bit, though, is the switch to clang.

When I was working on it before, there was a custom linker script that 
got used to produce loader as a PE binary (EFI uses the PE format and 
win32 ABI).  However, that script seemed to produce bad offsets when 
using clang.

On the other hand, clang can cross-compile to the win32 ABI (by giving 
it -target x86_64-unknown-win32).  This does have some additional 
advantages; win32 has slightly different conventions, and I remember 
reading about an issue someone was having because of a stack alignment 
issue when they were using a linker script solution.  So the thing to do 
might be to cross-compile loader and its dependencies (stand, ficl, if I 
recall).  The issue here is that you'd potentially be producing two sets 
of libraries: one with the standard ABI and one with the win32 ABI. 
Worth noting: there's supposedly a way to get clang to produce "object" 
files that are actually LLVM bitcode; that could potentially avoid 
generating two sets of object files.

I welcome any suggestions or comments.