[compiler-rt] [compiler-rt] Work around a warning from -Wgnu-anonymous-struct (PR #120314)

[Kazu Hirata via llvm-commits]

kazutakahirata wrote:

> Would it work to just add `__extension__` instead?

That works.  It's a lot simpler.  Thanks!

https://github.com/llvm/llvm-project/pull/120314