RFR: 8345659: Fix broken alignment after ReservedSpace splitting in GC code

[Stefan Karlsson]

The Serial and Parallel GCs create a ReservedSpace for the total heap and then splits it into a young generation ReservedSpace and an old generation ReservedSpace. The latter operation creates an ReservedSpace with an alignment that doesn't match the base address. This bug is benign because the ReservedSpaces are short-lived and we don't look at the alignment. However, if we are to add stricter checks when creating ReservedSpaces we need to fix this. 

Tested with tier1-3

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Commit messages:
 - 8345659: Fix broken alignment after ReservedSpace splitting in GC code

Changes: https://git.openjdk.org/jdk/pull/22602/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=22602&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8345659
  Stats: 2 lines in 2 files changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/22602.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/22602/head:pull/22602

PR: https://git.openjdk.org/jdk/pull/22602