CLEANUP: compiler: add a THREAD_ALIGNED macro and use it where appropriate

CLEANUP: compiler: add a THREAD_ALIGNED macro and use it where appropriate

Sometimes we need to align a struct member or a struct's size only when
threads are enabled. This is the case on fdtab for example. Instead of
using ugly ifdefs in the code itself, let's have a THREAD_ALIGNED() macro
performing the alignment only when threads are enabled. For now this was
only applied to fd-t.h as it was the only place found.