RFR: 8269924: Shenandoah: Introduce weak/strong marking asserts

[Roman Kennke]

On Tue, 6 Jul 2021 15:13:07 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

> For debugging, it is sometimes important to have not only `shenandoah_assert_marked`, but also `shenandoah_assert_marked_(weak|strong)` flavors.

Looks good.

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Marked as reviewed by rkennke (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/4691