[llvm-branch-commits] Reapply "[llvm][RISCV] Enable trailing fences for seq-cst stores by default (#87376)" (PR #90267)

[Paul Kirth via llvm-branch-commits]

https://github.com/ilovepi created https://github.com/llvm/llvm-project/pull/90267

With the tag merging in place, we can safely change the default for
+seq-cst-trailing-fence to the default, according to the recommendation in
https://github.com/riscv-non-isa/riscv-elf-psabi-doc/blob/master/riscv-atomic.adoc

This tag changes the default for the feature flag, and moves to more
consistent naming with respect to existing features.

This was reverted with https://github.com/llvm/llvm-project/pull/84597,
because ld.bfd would segfault with unknown riscv attributes.