std.tar: avoid dependency on file system

In the iterator function which is the low-level API, don't depend on
`std.fs.MAX_PATH_BYTES` because this is not defined on all operating
systems, such as freestanding.

However in such environments it still makes sense to be able to extract
from a tar file.

An even more flexible solution would be to accept the buffers as
arguments to iterator() which I think is a good idea, but for now let's
just set the same upper limmit across all operating systems.

lib/std/tar.zig

@@ -247,8 +247,8 @@ fn Iterator(comptime ReaderType: type) type {
 
         // buffers for heeader and file attributes
         header_buffer: [Header.SIZE]u8 = undefined,
-        file_name_buffer: [std.fs.MAX_PATH_BYTES]u8 = undefined,
-        link_name_buffer: [std.fs.MAX_PATH_BYTES]u8 = undefined,
+        file_name_buffer: [1024]u8 = undefined,
+        link_name_buffer: [1024]u8 = undefined,
 
         // bytes of padding to the end of the block
         padding: usize = 0,