[libc] Rename libc/src/__support/endian.h to endian_internal.h (#115950)

This prevents a conflict with the Linux system endian.h when built in
overlay mode for CPP files in __support.

This issue appeared in PR #106259.

GitOrigin-RevId: 95b680e4c353d479fbfb96adb39696042c005e99
15 files changed