Fix build warnings emitted by GCC 10 on Aarch64

[philn]

No description provided.

[miloyip]

This code is needed for sizeof(Ch) > 1. Otherwise the array lookup will be out of bound.
May need to add some pragma to disable warning.

[philn]

The value of unsigned vars can't be higher than 255 though. Instead of a pragma this might need some rework.

[miloyip]

Ch can be std::uint16_t or other types.

[ccawley2011]

PR #1895 adds the necessary pragmas to disable these warnings.