RFR: 8264346: nullptr_t undefined in global namespace for clang+libstdc++ [v2]

[Stefan Karlsson]

> There's a mismatch in some toolchains about what part should provide the nullptr_t definition. This patch takes the easy way out and include cstddef and changes the two usages nullptr_t to std::nullptr_t.
> 
> See the bug report for more details.
> 
> We could have redefined nullptr_t to resolve this, but that would have required more extensive testing, so I left that as a potential future cleanup.
> 
> I've tested this by compiling with clang on linux. I'm going to let GHA testing testing run, and will also run our tier1 testing.

Stefan Karlsson has updated the pull request incrementally with one additional commit since the last revision:

  Review Kim

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/3269/files
  - new: https://git.openjdk.java.net/jdk/pull/3269/files/5a562c14..600c8b71

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=3269&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=3269&range=00-01

  Stats: 4 lines in 2 files changed: 2 ins; 2 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3269.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3269/head:pull/3269

PR: https://git.openjdk.java.net/jdk/pull/3269