add trapframe structure for 64bit mode
don't compile entry page table in main in 64bit mode

It's not used by entry64.S and it fails to compile due 32bit-ness.
disable smp in 64bit mode for now
64bit support in exec.c and proc.c

- correct word size on the userspace startup stack
- correct argument passing for 64bit
- correct trapframe setup for 64bit
64bit vm/mmu and trap code
usertest: disable some assembly that needs 64bit-izing
makefile support for 32 and 64 bit builds

By default build for 32bit.  If X64 is defined, build for 63bit.
update notes