[build] Kill off the last multiple-object source file

The build mechanism currently allows for multiple objects per source
file.  The only remaining user of this is unnrv2b.S.  Replace this
usage with a separate unnrv2b16.S wrapper file, as is currently used
for e.g. pxeprefix.S and kpxeprefix.S.

src/arch/i386/Makefile

 NON_AUTO_SRCS	+= arch/i386/core/freebsd_loader.c
 NON_AUTO_SRCS	+= arch/i386/core/wince_loader.c
 
-# unnrv2b.S is used to generate a 16-bit as well as a 32-bit object.
-#
-OBJS_unnrv2b	= unnrv2b unnrv2b16
-CFLAGS_unnrv2b16 = -DCODE16
-
 # Include common x86 Makefile
 #
 MAKEDEPS	+= arch/x86/Makefile

src/arch/i386/prefix/unnrv2b16.S

+/*
+ * 16-bit version of the decompressor
+ *
+ */
+
+#define CODE16
+#include "unnrv2b.S"