Draft: Build with clang 19 on armhf instead of default clang 20

I'm setting as Draft since this doesn't affect Trixie or Unstable where the default clang is 19 not 20.

However, I am applying this in Ubuntu since Ubuntu 25.04 and 25.10 have the default clang as 20.

Ubuntu doesn't support armel. I'm guessing if armhf is broken, armel might be broken too.

The webkit bug is https://bugs.webkit.org/show_bug.cgi?id=290167 and the Ubuntu bug is https://launchpad.net/bugs/2103459

We need an upstream llvm/clang bug but I haven't reported that bug.

Maybe you'd want to apply this in experimental or maybe it's fine to just wait until Debian changes its default clang and then apply a workaround if needed at that time.