Do not define clear_stack_inner and next_random_no if not used

Do not define clear_stack_inner and next_random_no if not used
(code refactoring of commits 4eaea0afa, b850e1c9b)

* misc.c (SMALL_CLEAR_SIZE): Do not define if already defined; define
even if THREADS is not defined.
* misc.c [STACK_NOT_SCANNED] (GC_clear_stack): Standalone definition
(just return arg).
* misc.c (BIG_CLEAR_SIZE, GC_stack_last_cleared, GC_min_sp,
GC_high_water, GC_bytes_allocd_at_reset, DEGRADE_RATE,
GC_clear_stack_inner, next_random_no): Do not define if
STACK_NOT_SCANNED.
* misc.c [!STACK_NOT_SCANNED] (CLEAR_SIZE): Do not define if
ASM_CLEAR_CODE.
* misc.c (GC_stack_last_cleared, GC_min_sp, GC_high_water,
GC_bytes_allocd_at_reset, GC_clear_stack_inner, next_random_no,
GC_clear_stack): Adjust indentation.