x86, syscall: Allow syscall offset to be symbolic

Allow the specified syscall offset to be symbolic, e.g. a macro.  For
offset system calls, this if nothing else makes the generated code
easier to read.

Suggested-by: H. J. Lu <hjl.tools@gmail.com>
Link: http://lkml.kernel.org/r/1321569446-20433-7-git-send-email-hpa@linux.intel.com
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>