Enable 'Force GC at every GC_malloc' debug-related functionality

Enable 'Force GC at every GC_malloc' debug-related functionality

* doc/README.environment (GC_COLLECT_AT_MALLOC): Document.
* doc/README.macros (GC_COLLECT_AT_MALLOC): Likewise.
* fnlz_mlc.c (GC_core_finalized_malloc): Insert
GC_DBG_COLLECT_AT_MALLOC invocation (before LOCK).
* gcj_mlc.c (GC_core_gcj_malloc, GC_gcj_malloc_ignore_off_page):
Likewise.
* malloc.c (GC_generic_malloc, GC_malloc_atomic, GC_malloc,
GC_malloc_uncollectable): Likewise.
* mallocx.c (GC_generic_malloc_ignore_off_page,
GC_generic_malloc_many, GC_malloc_atomic_uncollectable): Likewise.
* typd_mlc.c (GC_malloc_explicitly_typed,
GC_malloc_explicitly_typed_ignore_off_page): Likewise.
* include/private/gc_priv.h (GC_COLLECT_AT_MALLOC): Recognize new
macro.
(GC_dbg_collect_at_malloc_min_lb): New global variable declaration
(only if GC_COLLECT_AT_MALLOC defined).
(GC_DBG_COLLECT_AT_MALLOC): Define new macro (invoking GC_gcollect).
* malloc.c (GC_dbg_collect_at_malloc_min_lb): New global variable
(only if GC_COLLECT_AT_MALLOC defined).
* misc.c (GC_init): Test "GC_COLLECT_AT_MALLOC" environment variable
and alter default GC_dbg_collect_at_malloc_min_lb value (only if
GC_COLLECT_AT_MALLOC macro defined).