Code-base clean-up and refactoring (#47)

* clean up the cpu.h abstraction

* refactor the pte copy algorithm for per-vmr copy

* draw a line between sys/ and asm/; make use of generic arch

restructure the arch/* directory to differentiate the functionality
of sys/ and asm/. In which the former refer to arch-specific kernel
service; the latter refer to code related to low-level architectural
feature.

make use the arch/generic which will serve as a supplier of default
implementation, to allow sharing common code across different arch.

* fix a missing return statement in usr/test_pthread.c

* clean up redundant header files, add default for hwtimer selection

* add documentation on porting to other ISAs

* move asm/isrm to asm-generic for sharing the header

* remove redundancy in syscall numbering