Add #include_next.
Fix #include for absolute path.

#include </stdio.h>, for example, shouldn't look for
stdio.h from the standard search paths.
Sort.
Initial commit for the new backend.
Compile a dumb simple program. No register allocation yet.

It's now able to compile

  int main() { int x = 2; return x + 5; }
Implement very simple register allocator.

It reserves stack space for all intermediate results.
It always spills an intermediate result when we run out of registers
even if the result will never be needed again. (In order to elminate
such stores, we need liveness analysis.)

Currently it assumes all variables are of type int.
Support assignment, array and pointer arithmetics.