RFR: 8271225: Add floorDivExact() method to java.lang.[Strict]Math

[Brian Burkhalter]

Add methods `floorDivExact(int,int)` and `floorDivExact(long,long)` to `java.lang.Math` and `java.lang.StrictMath`.

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Commit messages:
 - 8271225: Add floorDivExact() method to java.lang.[Strict]Math

Changes: https://git.openjdk.java.net/jdk/pull/4941/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4941&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8271225
  Stats: 245 lines in 3 files changed: 236 ins; 0 del; 9 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4941.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4941/head:pull/4941

PR: https://git.openjdk.java.net/jdk/pull/4941