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

[Albert Mingkun Yang]

On Fri, 6 Dec 2024 10:16:33 GMT, Stefan Karlsson <stefank at openjdk.org> wrote:

> 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

I wonder if doing the same for `first_part` makes it more symmetric/readable.

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PR Review: https://git.openjdk.org/jdk/pull/22602#pullrequestreview-2495045065