RFR: 8250563: Add KVHashtable::add_if_absent [v2]

[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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  8250563: Add KVHashtable::add_if_absent

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/66/files
  - new: https://git.openjdk.java.net/jdk/pull/66/files/f41faaad..5249b6be

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=66&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=66&range=00-01

  Stats: 0 lines in 0 files changed: 0 ins; 0 del; 0 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