Generate <asm/unistd_x32.h>; this exports x32 system call numbers to
user space.

[ v2: Enclose all arguments to syshdr in '' so empty arguments aren't
      dropped on the floor. ]
Signed-off-by: NH. Peter Anvin <hpa@zytor.com>

Split the 64-bit system calls into "64" (64-bit only) and "common"
(64-bit or x32) and add the x32 system call numbers.
Signed-off-by: NH. Peter Anvin <hpa@zytor.com>

Create a simple set of syscall tables and scripts to turn them into
both header files (unistd_*.h) and macros for generating the system
call tables.
Signed-off-by: NH. Peter Anvin <hpa@linux.intel.com>