[Bug target/80881] Implement Windows native TLS

[lh_mouse at 126 dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80881

--- Comment #101 from LIU Hao <lh_mouse at 126 dot com> ---
I have bootstrapped GCC 15 (master) with this patch applied, but without
`--enable-tls`, on {i686,x86_64}-w64-mingw32; and have checked that TLS is
emulated as before so it's not an ABI break by default.

I'm preparing more builds with `--enable-tls` now.