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

[Aleksey Shipilev]

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

-------------

Commit messages:
 - 8269924: Shenandoah: Introduce weak/strong marking asserts

Changes: https://git.openjdk.java.net/jdk/pull/4691/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4691&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8269924
  Stats: 46 lines in 2 files changed: 46 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4691.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4691/head:pull/4691

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