[Bug target/120933] Turn on -mtls-dialect=gnu2 by default on x86-64

[hjl.tools at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=120933

--- Comment #2 from H.J. Lu <hjl.tools at gmail dot com> ---
This should be opt-in since it requires glibc fix

https://sourceware.org/bugzilla/show_bug.cgi?id=31372