RFR: JDK-8300080: offset_of for GCC/Clang exhibits undefined behavior and is not always a compile-time constant [v3]

[Justin King]

> The implementation of `offset_of` for GCC/Clang only deals with types are aligned to 16 bytes or less, if they are more, such as `zCollectedHeap` the behavior is undefined. UBSan also suggests that `offset_of` is not always a compile time constant, as the stack trace came from the dynamic loader during library loading. This patch changes `offset_of` to use `offsetof` and disables the warning `invalid-offsetof` for the JVM.

Justin King has updated the pull request incrementally with one additional commit since the last revision:

  Use parenthesis
  
  Signed-off-by: Justin King <jcking at google.com>

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Changes:
  - all: https://git.openjdk.org/jdk/pull/11978/files
  - new: https://git.openjdk.org/jdk/pull/11978/files/3f46a86b..8d0b73c8

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=11978&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=11978&range=01-02

  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/11978.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11978/head:pull/11978

PR: https://git.openjdk.org/jdk/pull/11978