Due to some changes in the headers, racket doesn't build on OpenBSD -current (the future OpenBSD 5.6). This patch fixes the problem.

Due to some changes in the headers, racket doesn't build on OpenBSD
-current (the future OpenBSD 5.6). This patch fixes the problem.

Additional changes:
- Fix the mix of spaces and tabs in the indentation of OpenBSD blocks.
- Remove the old GC_OPENBSD_THREADS. OpenBSD doesn't use this feature
  anymore.
- Add comments to the end of the block to easy the reading of the code.
- Change the hardcoded aligment values of MIPS to _MIPS_SZPTR.
- Remove HEURISTIC2 from the latest x86_64 block because racket picks
  STACKBOTTOM before and never uses HEURISTIC2.
- Change GETPAGESIZE() to use sysconf(_SC_PAGESIZE).