ld: don't hardcode "-melf_i386"

On FreeBSD, the name of the emulation is "-melf_i386_fbsd", so allow
SeaBIOS to fetch the emulation to use from the environment variables.

This allows SeaBIOS to compile on FreeBSD with gcc.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
diff --git a/Makefile b/Makefile
index e567ba1..4338f94 100644
--- a/Makefile
+++ b/Makefile
@@ -24,6 +24,7 @@
 PYTHON=python
 CPP=cpp
 IASL:=iasl
+LD32BIT_FLAG:=-melf_i386
 
 # Source files
 SRCBOTH=misc.c stacks.c output.c string.c x86.c block.c cdrom.c mouse.c kbd.c \
@@ -154,8 +155,8 @@
 	@echo "  Building ld scripts"
 	$(Q)./scripts/buildversion.sh $(OUT)version.c
 	$(Q)$(CC) $(CFLAGS32FLAT) -c $(OUT)version.c -o $(OUT)version.o
-	$(Q)$(LD) -melf_i386 -r $(OUT)ccode32flat.o $(OUT)version.o -o $(OUT)code32flat.o
-	$(Q)$(LD) -melf_i386 -r $(OUT)ccode16.o $(OUT)romlayout.o -o $(OUT)code16.o
+	$(Q)$(LD) $(LD32BIT_FLAG) -r $(OUT)ccode32flat.o $(OUT)version.o -o $(OUT)code32flat.o
+	$(Q)$(LD) $(LD32BIT_FLAG) -r $(OUT)ccode16.o $(OUT)romlayout.o -o $(OUT)code16.o
 	$(Q)$(OBJDUMP) -thr $(OUT)code32flat.o > $(OUT)code32flat.o.objdump
 	$(Q)$(OBJDUMP) -thr $(OUT)code32seg.o > $(OUT)code32seg.o.objdump
 	$(Q)$(OBJDUMP) -thr $(OUT)code16.o > $(OUT)code16.o.objdump