x86: Makefile: Fix linkage of realmode on x86_64-elf binutils

link spec [1][2] is empty on x86_64-elf-gcc, i.e. -m32 is not propogated
to the linker as "-m elf_i386" and that causes the error:

  /usr/local/opt/x86_64-elf-binutils/bin/x86_64-elf-ld: i386 architecture
  of input file `x86/realmode.o' is incompatible with i386:x86-64 output

1. https://gcc.gnu.org/onlinedocs/gcc/Spec-Files.html
2. https://gcc.gnu.org/onlinedocs/gccint/Driver.html



Cc: Cameron Esfahani <dirty@apple.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Roman Bolshakov <r.bolshakov@yadro.com>
Message-Id: <20200901085056.33391-4-r.bolshakov@yadro.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>