std.zig.system.NativeTargetInfo: improve examined file search

[BratishkaErik]

• Append Termux-specific path to zig_examined_elf_files to workaround different path for env binary. See also Compiling a program on termux aarch64 produces FileNotFound errors #14146

• Use same buffer for searching shebang line to search ELF magic constant, if any, and remove corresponding (now duplicate) check from abiAndDynamicLinkerFromFile

• Fix not finding interpret path when it it separater from shebang by one or more blank spaces

• Fix too big minimal limit of bytes from file (minimal shebang length instead of buffer.len)

BTW with this changes Zig correctly chases files from shebang line again, but abiAndDynamicLinkerFromFile returns incorrect result 2.19.0 instead of 2.37 (hidden bug popped up), so this PR is not enough to mitigate all related issues.

[BratishkaErik]

I forgot to add, it is an alternative to #15896, mentioned in comment #15896 (comment)

[BratishkaErik]

As per discussion in Discord: environment variable was removed. Instead, Termux-specific path is hardcoded in that variable.
https://github.com/ziglang/zig/compare/acdc768849dc61a2e853f10a51a3f1d559daba71..afb3fb366414c7e400547a61c4ceb76dca85e874

-    const zig_examined_elf_files_envvar = std.os.getenv("ZIG_EXAMINED_ELF_FILES") orelse "/usr/bin/env";
-    var examined_elf_file_names_iterator = mem.tokenizeScalar(u8, zig_examined_elf_files_envvar, fs.path.delimiter);
+    const zig_examined_elf_files = std.fmt.comptimePrint("/usr/bin/env{[0]}/data/data/com.termux/files/usr/bin/env", .{fs.path.delimiter});
+    var examined_elf_file_names_iterator = mem.tokenizeScalar(u8, zig_examined_elf_files, fs.path.delimiter);

[BratishkaErik]

I can't reproduce errors at all...

[BratishkaErik]

I'll close it for a bit while kubkon upstreams his zld/elf linker. It's not blocking me, just that in my following commit (with fixing CI failure) I also deduplicated (and sometimes duplication is incomplete) std.elf logic in abi-parser functions here, and I don't know what will be changed in std.elf after upstreaming, so I'll probably need to redo deduplication work anyway.