Allow configuring for a given boot platform.
ByteOrder.h: remove use of __builtin_bswap16.

* Fixes the build for host compilers older than gcc-4.8

Conflicts:
	headers/os/support/ByteOrder.h
Revert "ByteOrder.h: remove use of __builtin_bswap16."

This reverts commit 040dc2eebc6628cc05c4ac5e287c5287c5dacf2e.
ByteOrder.h: separate host & Haiku versions.

* This avoids polluting the Haiku headers with host issues,
  as pointed out by Axel.
* Should also resolve build issues for various versions of
  host compilers that were introduced in previous commits.