x86: Makefile: Allow division on x86_64-elf binutils

For compatibility with other SVR4 assemblers, '/' starts a comment on
*-elf binutils target and thus division operator is not allowed [1][2].
That breaks cstart64.S build:

  x86/cstart64.S: Assembler messages:
  x86/cstart64.S:294: Error: unbalanced parenthesis in operand 1.

configure should detect if --divide needs to be passed to assembler by
compiling a small snippet where division is used inside parentheses.

1. https://sourceware.org/binutils/docs/as/i386_002dChars.html
2. https://sourceware.org/binutils/docs/as/i386_002dOptions.html#index-_002d_002ddivide-option_002c-i386



Cc: Cameron Esfahani <dirty@apple.com>
Signed-off-by: Roman Bolshakov <r.bolshakov@yadro.com>
Message-Id: <20200901085056.33391-2-r.bolshakov@yadro.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>