cmd/asm,cmd/internal/obj/riscv: update references to RISC-V specification

Update section numbers and names to align with the 20260120 version of the
RISC-V Instruction Set Manual. The majority of the change is adjustments
to section numbers. Zbb collapses into a single section and Vector Strided
is renamed to Vector Constant-Stride. Also use consistent sections for
vector segment loads and stores.

Change-Id: Ie425b36ea5d253d3f64d35e2787eeb553fd95d20
Reviewed-on: https://go-review.googlesource.com/c/go/+/762240
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Mark Ryan <markdryan@meta.com>
Reviewed-by: Meng Zhuo <mengzhuo1203@gmail.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>
Auto-Submit: Joel Sing <joel@sing.id.au>