RFR: 8250563 Add KVHashtable::add_if_absent

[Ioi Lam]

Please review this XS change. I added a new **KVHashtable::add_if_absent** function (modeled after
ResourceHashtable::put_if_absent from JDK-8244733).

- I used "add" instead of "put" to be consistent with the naming convention in utility/hashtable.hpp
- I also fixed a type in the comments in resourceHashtable.hpp

Running mach5 tiers1/2.

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Commit messages:
 - 8250563 Add KVHashtable::add_if_absent

Changes: https://git.openjdk.java.net/jdk/pull/66/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=66&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8250563
  Stats: 33 lines in 4 files changed: 21 ins; 3 del; 9 mod
  Patch: https://git.openjdk.java.net/jdk/pull/66.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/66/head:pull/66

PR: https://git.openjdk.java.net/jdk/pull/66