Improve logged messages about heap size and usage

Improve logged messages about heap size and usage

* alloc.c (GC_stopped_mark): Print heap size to debug log in KiB
(using TO_KiB_UL macro) excluding unmapped memory size.
* alloc.c (GC_compute_heap_usage_percent): New inline function.
* alloc.c (GC_finish_collection): Use GC_DBGLOG_PRINTF instead of
GC_COND_LOG_PRINTF for heap usage logging; print heap usage in KiB;
log memory heap utilization in percent (using
GC_compute_heap_usage_percent).
* alloc.c (GC_expand_hp_inner): Log increased heap size (in KiB)
instead of increment size.
* include/private/gc_priv.h (TO_KiB_UL): New macro.