gas: add missing header guard in tc-<arch>.h files

This patch adds missing header guards in some of the tc-<arch>.h,
and merely comments on the corresponding #endif for others. The
patch does not aim at being exhaustive, it only touched the files
relevant for [1].

[1]: https://inbox.sourceware.org/binutils/20250711112913.2453285-1-matthieu.longo@arm.com/