RFR: 8253206: Enforce whitespace checking for additional source files

[Kevin Rushforth]

This adds the following extensions to the list of source files that git jcheck will check for whitespace errors:

 .cc, .hh, .m, .mm

All files with the above extensions are now white-space clean after the fix for
[JDK-8240487](https://bugs.openjdk.java.net/browse/JDK-8240487). This will help them stay that way.

I just integrated a similar fix to `jfx` (with a few more source extensions that aren't relevant for the JDK) in
[JDK-8240499](https://bugs.openjdk.java.net/browse/JDK-8240499).

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Commit messages:
 - 8253206: Enforce whitespace checking for additional source files

Changes: https://git.openjdk.java.net/jdk/pull/195/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=195&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8253206
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/195.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/195/head:pull/195

PR: https://git.openjdk.java.net/jdk/pull/195