windows: _tls_index linker error fixes

[kcbanner]

This should fix #8531 and #13549, which should fixup the CI for #13562 (which exposed this error).

There were three issues here:

• On x86, the name is mangled to __tls_index
• As per this comment (LLD with LTO enabled incorrectly drops _tls_index symbol, causing undefined reference errors #8531 (comment)), the logic needs to happen even if we aren't linking libc. _tls_index is provided by start_windows_tls.zig in this case
• Don't do this if we're single-threaded, because start_windows_tls.zig won't be included

[jedisct1]

Yup, that fix works.

[xxxbxxx]

nice!
good catch with the extra _