…in/linker*` to bypass android app data file exec restriction if using `targetSdkVersion` `>= 28` on Android `>= 10`

- See the README file for a description for who `system_linker_exec` works.
- If `TERMUX_EXEC_OPTOUT` environment variable is set, then do not intercept `execve()` at all to fix shebang issues or use `system_linker_exec`.
- If `TERMUX_EXEC_DEBUG` environment variable is set, then log debug info for `termux-exec`.
- Use `TERMUX__PREFIX` environment variable to extract termux rootfs directory path that's used to generate termux bin path to replace `/bin` and `/usr/bin` path in shebang of scripts instead of using hardcoded `TERMUX_BASE_DIR` build variable.
- Since when executing with linker, the `/proc/self/exe` will be set to linker path, export `TERMUX_EXEC__PROC_SELF_EXE` environment variable with actual path to executable being executed so that packages can be patched to read it instead.
- Added `src/exec-variants.c` to hook the entire `exec()` family of functions, which is required for Android 14. Closes termux/termux-packages#18537, termux/termux-app#3758