elf: pad out file to the required size when init data

We need to pad out the file to the required maximum size equal the
final section's offset plus the section's size. We only need to
this when populating initial metadata and only when section header
was updated.

src/link/Elf.zig

@@ -922,6 +922,21 @@ pub fn populateMissingMetadata(self: *Elf) !void {
         }
         // We are starting with an empty file. The default values are correct, null and empty list.
     }
+
+    if (self.shdr_table_dirty) {
+        // We need to find out what the max file offset is according to section headers.
+        // Otherwise, we may end up with an ELF binary with file size not matching the final section's
+        // offset + it's filesize.
+        var max_file_offset: u64 = 0;
+
+        for (self.sections.items) |shdr| {
+            if (shdr.sh_offset + shdr.sh_size > max_file_offset) {
+                max_file_offset = shdr.sh_offset + shdr.sh_size;
+            }
+        }
+
+        try self.base.file.?.pwriteAll(&[_]u8{0}, max_file_offset);
+    }
 }
 
 pub const abbrev_compile_unit = 1;