[Issue 21393] core.thread.fiber: ignores RAM allocation if mmap/valloc/malloc not found

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=21393

--- Comment #1 from feklushkin.denis at gmail.com ---
Addition: this is about Posix only.

malloc will never be found, even if it is listed in the stdlib module, because
this function also isn't imported

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