After conffigure is run I have the following in make.defaults:

# LINKER COMMANDS

LIB_LINK:= CC $(GLOB_OPTIONS)
STATICLIB_LINK:= ar cruvs
CLIENTLIB_LINK:= cc -xc99=all $(GLOB_OPTIONS)
EXE_LINK:= CC $(GLOB_OPTIONS)
STATICEXE_LINK:= CC $(GLOB_OPTIONS)

The problem is that both cc and CC assume -m32 memory model by default and when building 64bit binaries linker ld tries to generate 32bit object off 64bit objects. The following is reported:

ld: fatal: file ../temp/boot/jrd/dsc.o: wrong ELF class: ELFCLASS64

and the compilation breaks.

It can be easily fixed by the following change in make.defaults:

LIB_LINK:= CC $(GLOB_OPTIONS) $(CXXFLAGS)
CLIENTLIB_LINK:= cc -xc99=all $(GLOB_OPTIONS) $(CFLAGS)
EXE_LINK:= CC $(GLOB_OPTIONS) $(CXXFLAGS)
STATICEXE_LINK:= CC $(GLOB_OPTIONS) $(CXXFLAGS)