x86: Replace instruction prefixes with spaces

There are three kinds of x86 prefix delimiters in GNU binutils:
'/', '\\' and a space.

The first works on Linux and few other platforms.  The second one is
SVR-4 compatible and works on the generic elf target. The last kind is
universal and works everywhere, it's also used in the GAS manual [1].
Space delimiters fix the build errors on x86_64-elf binutils:

  x86/cstart64.S:217: Error: invalid character '/' in mnemonic
  x86/cstart64.S:313: Error: invalid character '/' in mnemonic

1. https://sourceware.org/binutils/docs/as/i386_002dPrefixes.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: <20200901085056.33391-3-r.bolshakov@yadro.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>