RFR: 8377658: G1: Cleanup g1ConcurrentMark.cpp for stricter warning flags

[Leo Korinth]

Cleanup `g1ConcurrentMark.cpp` and included header files to remove implicit narrowing conversions.

For gcc: `-Wconversion -Wno-float-conversion`
For clang: `-Wimplicit-int-conversion`

`sizeof_auto` is created so that we can reduce the amount of casting and thus make the code more type safe. The normal `sizeof` will return a `size_t` although the size of most stuff can be represented in a `uint8_t`. `sizeof_auto`  will return the size in an as small unsigned type as is possible. The result is that expressions that uses `sizeof_auto` can convert to most integral types, and no explicit narrowing cast will be needed.

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Commit messages:
 - 8377658: G1: Cleanup g1ConcurrentMark.cpp for stricter warning flags

Changes: https://git.openjdk.org/jdk/pull/29701/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29701&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8377658
  Stats: 74 lines in 11 files changed: 52 ins; 0 del; 22 mod
  Patch: https://git.openjdk.org/jdk/pull/29701.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29701/head:pull/29701

PR: https://git.openjdk.org/jdk/pull/29701