Patch created by John L. Poole 3/1/2018
Referencing Gentoo Bugs: 
#602052 - app-emulation/xen-4.8.2-r1 GRUB_PLATFORMS="efi-64" fails to build: Makefile:141: recipe for target fails
#649048 - app-emulation/xen-4.9.1-r1 fails to compile of UEFI motherboard 

The method for creating this patch was the same as I used for the patch 
for 4.8.2-r1 available in Bug #602052.  I went in and hand edited three
lines adding a flag "-b coff-x86-64".  Commentator  Pat Erley on 
Bug #602052 at 2017-01-08 08:02:56 UTC 
https://bugs.gentoo.org/602052#c19
noted my choice of coff-x86-64
vs. PE.  I have no justification for choosing  coff-x86-64; it was the
first I tried and using it allowed me to move forward.  I leave the
merits of using PE vs. coff-x86-64 for someone else to devine.

If I recall correctly, I used
that patch (https://bugs.gentoo.org/attachment.cgi?id=455684)
in a portage overlay ("Portage Overlay"). That is more cumbersome, in 
my opinion and adds complexity to emerge updates because I have something
custom lurking on my system that could be interfering with an upgrade.
  
This time, I am using the feature of user patches ("User Patches") 
by placing a patch under:
/etc/portage/patches/app-emulation/xen

Emerge kept dying saying that it could not find the files.  So with
some midnight oil, I determined that the call to patch uses the "-p1"
parameter which strips off the first forward slash.  When I tried
to use the same form as in the previous patch for xen-4.8.2-r1, namely
specifying the file path with "xen/arch...", emerge kept failing at
the patch point because "patch" claimed it could not find the file.
I created a study re: patching and have shared it at:
https://forums.gentoo.org/viewtopic-t-1077806.html as "Sample Patch Study"

In order for this patch to succeed in a User Patches mode, I had to
prefix "xen/arch..." with a forward slash, "/", as you can see below.
Then when emerge tried to apply this patch with the "-p1" flag, 
the patch succeeded.

WARNING: I suspect this patch will not work in the Portage Overlay 
method because of the presence of the forward slash, so don't use this
patch for a Portage Overlay approach.  However, you may be able
to simply modify this patch to work for a Portage Overlay by simply
removing the two forward slash characters.

The real heart of the matter appears to be with binutils determining
there is an ambiguity and croaking.  Until that is fixed, this patch
approach seems to be the only way to build Xen on a EFI based system.

For improvements on the form of this patch: 

see scholarly and quirky: https://dev.gentoo.org/~vapier/clean-patches
    
Bug #649048 was marked as a duplicate, but since this patch is using
the User Patches approach instead of Bug #602052 Portage Overlay approach,
I'm placing this patch in Bug #649048.

--- /xen/arch/x86/Makefile_ORIGINAL	2018-02-28 21:23:49.632816030 -0800
+++ /xen/arch/x86/Makefile	2018-02-28 21:30:58.474793635 -0800
@@ -177,20 +177,20 @@
 $(TARGET).efi: prelink-efi.o $(note_file) efi.lds efi/relocs-dummy.o $(BASEDIR)/common/symbols-dummy.o efi/mkreloc
 	$(foreach base, $(VIRT_BASE) $(ALT_BASE), \
 	          $(guard) $(LD) $(call EFI_LDFLAGS,$(base)) -T efi.lds -N $< efi/relocs-dummy.o \
-	                $(BASEDIR)/common/symbols-dummy.o $(note_file) -o $(@D)/.$(@F).$(base).0 &&) :
+	                $(BASEDIR)/common/symbols-dummy.o -b coff-x86-64 $(note_file) -o $(@D)/.$(@F).$(base).0 &&) :
 	$(guard) efi/mkreloc $(foreach base,$(VIRT_BASE) $(ALT_BASE),$(@D)/.$(@F).$(base).0) >$(@D)/.$(@F).0r.S
 	$(guard) $(NM) -pa --format=sysv $(@D)/.$(@F).$(VIRT_BASE).0 \
 		| $(guard) $(BASEDIR)/tools/symbols $(all_symbols) --sysv --sort >$(@D)/.$(@F).0s.S
 	$(guard) $(MAKE) -f $(BASEDIR)/Rules.mk $(@D)/.$(@F).0r.o $(@D)/.$(@F).0s.o
 	$(foreach base, $(VIRT_BASE) $(ALT_BASE), \
 	          $(guard) $(LD) $(call EFI_LDFLAGS,$(base)) -T efi.lds -N $< \
-	                $(@D)/.$(@F).0r.o $(@D)/.$(@F).0s.o $(note_file) -o $(@D)/.$(@F).$(base).1 &&) :
+	                $(@D)/.$(@F).0r.o $(@D)/.$(@F).0s.o -b coff-x86-64 $(note_file) -o $(@D)/.$(@F).$(base).1 &&) :
 	$(guard) efi/mkreloc $(foreach base,$(VIRT_BASE) $(ALT_BASE),$(@D)/.$(@F).$(base).1) >$(@D)/.$(@F).1r.S
 	$(guard) $(NM) -pa --format=sysv $(@D)/.$(@F).$(VIRT_BASE).1 \
 		| $(guard) $(BASEDIR)/tools/symbols $(all_symbols) --sysv --sort >$(@D)/.$(@F).1s.S
 	$(guard) $(MAKE) -f $(BASEDIR)/Rules.mk $(@D)/.$(@F).1r.o $(@D)/.$(@F).1s.o
 	$(guard) $(LD) $(call EFI_LDFLAGS,$(VIRT_BASE)) -T efi.lds -N $< \
-	                $(@D)/.$(@F).1r.o $(@D)/.$(@F).1s.o $(note_file) -o $@
+	                $(@D)/.$(@F).1r.o $(@D)/.$(@F).1s.o -b coff-x86-64 $(note_file) -o $@
 	if $(guard) false; then rm -f $@; echo 'EFI support disabled'; \
 	else $(NM) -pa --format=sysv $(@D)/$(@F) \
 		| $(BASEDIR)/tools/symbols --xensyms --sysv --sort >$(@D)/$(@F).map; fi