Adjust GC_debug_malloc/realloc_replacement macro to match the API function

* malloc.c (GC_debug_malloc_replacement): Pass NULL (instead of
"unknown") to "s" argument of GC_debug_malloc to match that
replacement definition in dbg_mlc.c.
* mallocx.c (GC_debug_realloc_replacement): Likewise.
Replace GC_DBG_RA with GC_DBG_EXTRAS macro

* dbg_mlc.c (GC_debug_malloc_replacement,
GC_debug_realloc_replacement): Use GC_DBG_EXTRAS (instead of
"GC_DBG_RA NULL, 0").
* malloc.c (GC_debug_malloc_replacement): Likewise.
* mallocx.c (GC_debug_realloc_replacement): Likewise.
* include/private/gc_priv.h (GC_DBG_EXTRAS): New macro (similar to
GC_EXTRAS but pass NULL/0 instead of __FILE__and __LINE__);
pass "unknown" instead of NULL unless GC_ADD_CALLER.
* include/private/gc_priv.h (GC_DBG_RA): Remove.
Define GC_caller_func_offset only if HAVE_DLADDR and GC_RETURN_ADDR_PARENT

* dbg_mlc.c (GC_caller_func_offset): Do not define (and do not include
dlfcn.h) unless GC_ADD_CALLER; test HAVE_DLADDR (and
GC_RETURN_ADDR_PARENT) instead of FREEBSD macro.
* dbg_mlc.c (GC_caller_func_offset): Use STATIC; replace the K&R-style
function definition with the ANSI C one; remove unnecessary "const" in
type casts; do not call dladdr() if address is 0.
* dbg_mlc.c (GC_debug_malloc, GC_debug_realloc): Move assignment of
"s" argument to "unknown" to GC_caller_func_offset (if GC_ADD_CALLER).
* dbg_mlc.c (GC_debug_realloc): Do not call GC_caller_func_offset if
"p" argument is NULL (i.e., if redirected to GC_debug_malloc)..
* include/private/gcconfig.h (HAVE_DLADDR): Define for FREEBSD.
Prevent use of unsupported __builtin_return_address(1) in GC_DBG_EXTRAS

* include/gc_config_macros.h (GC_CAN_SAVE_CALL_STACKS): Define also
for __amd64__ (synonym for __x86_64__).
* include/gc_config_macros.h (GC_RETURN_ADDR_PARENT): Define only for
x86 and x86_64 (which have frame-pointer register, for other targets
GCC might report "unsupported argument to __builtin_return_address"
warning).