RFR: 8081833: Clean up JVMFlag getter/setter code

[Ioi Lam]

This is a follow to [JDK-8243208 Clean up JVMFlag implementation](https://bugs.openjdk.java.net/browse/JDK-8243208).

I try to use templates and inheritance (and a bit of macros) to get rid of the large amount of duplicated code in the
JVMFlag getters/setters.

Stefan Karlsson and I have tried this several times already, but I think the current attempt probably has the least
amount of hacks and most functionality:

- Type safety is enforced up to the point of loading/storing `JVMFlag::_addr`, without any unchecked typecasts.
- Runtime type checking is implemented by the `JVM_FLAG_TYPE(t)` macro (see jvmFlagAccess.hpp), by passing along an
  integer (`JVMFlag::TYPE_int`, etc). This is more efficient than previous attempts that passed along a function pointer
  (see http://cr.openjdk.java.net/~stefank/8081833/webrev.04/)
- The old `JVMFlag::xxxAtPut` functions were undocumented. There are actually two groups that have different usage. I
  added documentation in jvmFlagAccess.hpp to explain this.
- I got rid of the `JVMFlagEx` class and moved the code into `JVMFlag`. This is possible with C++11 which allows forward
  declaration of `enum JVMFlagsEnum : int;`
- I changed `JVMFlag::_type` from a string into an enum. I am surprised this had lasted for over 20 years.
- I also changed `JVMFlag` from a struct into a class and made some fields private. I probably will do more cleanup in a
  separate RFE.

Please start with jvmFlagAccess.hpp, jvmFlagAccess.hpp and then globals_extension.hpp.

Probably not everyone is in love with the `JVM_FLAG_TYPE(t)` macro. I'll be happy to try out alternatives.

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Commit messages:
 - 8081833: Clean up JVMFlag getter/setter code

Changes: https://git.openjdk.java.net/jdk/pull/163/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=163&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8081833
  Stats: 2260 lines in 26 files changed: 668 ins; 1397 del; 195 mod
  Patch: https://git.openjdk.java.net/jdk/pull/163.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/163/head:pull/163

PR: https://git.openjdk.java.net/jdk/pull/163