RFR: 8337563: NMT: rename MEMFLAGS to MemFlag

[Gerard Ziemski]

Please review this cleanup, where we rename `MEMFLAGS` to `MemType`.

`MEMFLAGS` implies that we can use more than one at the same time, but those are exclusive values, so `MemType` is much more suitable name.

There is a bunch of other related cleanup that we can do, but I will leave for follow up issues such as [NMT: rename NMTUtil::flag to NMTUtil::type](https://bugs.openjdk.org/browse/JDK-8337836)

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Commit messages:
 - rename MEMFLAGS to MemType

Changes: https://git.openjdk.org/jdk/pull/20497/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20497&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8337563
  Stats: 502 lines in 100 files changed: 1 ins; 0 del; 501 mod
  Patch: https://git.openjdk.org/jdk/pull/20497.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20497/head:pull/20497

PR: https://git.openjdk.org/jdk/pull/20497