[llvm][RISCV] Enable trailing fences for seq-cst stores by default (#87376)

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.