[Issue 23483] add profile-gc hook function user can call early to dump partial report file.

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=23483

--- Comment #1 from mw <mingwu at gmail.com> ---
Also, currently in the report profilegc.log only the call stack top allocation
site is reported, this is not very useful for things like containers from the
common library, e.g. array:

void[] std.array.Appender!(char[][]).Appender.ensureAddable.__lambda9

It's much informative to give the full (or top 20 stack traces) to see where 
the allocation happened in the user code, as what valgrind does.

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