RFR: 8364019: Add alignment precondition to Universe::reserve_heap

[Albert Mingkun Yang]

Simple converting a redundant `align_up` to assert.

Test: tier1

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Commit messages:
 - heap-align-precondition

Changes: https://git.openjdk.org/jdk/pull/26456/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26456&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8364019
  Stats: 2 lines in 1 file changed: 1 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/26456.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26456/head:pull/26456

PR: https://git.openjdk.org/jdk/pull/26456