crossgcc: Fix x86_64-elf target's -m16 support

It still needs to pass --32 (yes, 32) to the assembler.
x86_64-linux does this (through some other config file),
x86_64-elf did not.

This fixes building SeaBIOS with our x86_64-elf multilib compiler.

Change-Id: Ibe2a70e46e64e71c947482be5ec0eaf7f7bf300d
Signed-off-by: Patrick Georgi <patrick@georgi-clan.de>
Reviewed-on: http://review.coreboot.org/11289
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>

util/crossgcc/patches/gcc-5.2.0_elf_biarch.patch

@@ -74,3 +74,14 @@ diff -urN gcc-4.9.2/gcc/config.gcc gcc-4.9.2/gcc/config.gcc
  	;;
  i[34567]86-*-rdos*)
      tm_file="${tm_file} i386/unix.h i386/att.h dbxelf.h elfos.h newlib-stdint.h i386/i386elf.h i386/rdos.h"
+--- gcc-5.2.0/gcc/config/i386/x86-64.h.orig	2015-08-20 17:17:34.555919593 +0200
++++ gcc-5.2.0/gcc/config/i386/x86-64.h	2015-08-20 17:17:42.615908670 +0200
+@@ -49,7 +49,7 @@
+ #define WCHAR_TYPE_SIZE 32
+ 
+ #undef ASM_SPEC
+-#define ASM_SPEC "%{m32:--32} %{m64:--64} %{mx32:--x32}"
++#define ASM_SPEC "%{m16|m32:--32} %{m64:--64} %{mx32:--x32}"
+ 
+ #undef ASM_OUTPUT_ALIGNED_BSS
+ #define ASM_OUTPUT_ALIGNED_BSS(FILE, DECL, NAME, SIZE, ALIGN) \