fix struct stat

[arnetheduck]

Undo damage of #14170

[github-actions]

Thanks for your hard work on this PR!
The lines below are statistics of the Nim compiler built from 4b76129

Hint: mm: orc; opt: speed; options: -d:release
168057 lines; 8.745s; 612.219MiB peakmem

[c-blake]

As done here, this program no longer compiles:

import posix, tables

type Foo = object
  bar: Stat
  x: string

proc foo() =
  var f = initTable[int, Foo](4)
  f[0] = Foo()

foo()

because the generated C refers to a not defined reserved field of the C struct (It's called __glibc_reserved for me). There are also problems with many other tables procs, and presumably many other uses of Stat.

@Araq mentioned in #14170 needing a new {.padding.} or {.hidden.} annotation, also mentioned in the RFC mentioned there. There may be a more specific workaround here, but you might consider rolling back this change until it is fixed.