Port struct Lisp_FLoat to oddball platforms

Port struct Lisp_FLoat to oddball platforms

* src/lisp.h (struct Lisp_Float): Declare via
GCALIGNED_UNION_MEMBER, not via GCALIGNED_STRUCT, since alloc.c
creates these in arrays and GCALIGNED_STRUCT does not necessarily
suffice to align struct Lisp_Float when it’s used in an array.
This avoids undefined behavior on oddball machines where
sizeof (struct Lisp_Float) is not a multiple of 8 and the compiler
does not support __attribute__ ((aligned 8)).