cmd/compile,bytealg: change context register on riscv64

The register ABI will use X8-X23 (CL 356519),
this CL changes context register from X20(S4) to X26(S10) to meet the
prerequisite.

Update #40724

Change-Id: I93d51d22fe7b3ea5ceffe96dff93e3af60fbe7f6
Reviewed-on: https://go-review.googlesource.com/c/go/+/357974


Trust: mzh <mzh@golangcn.org>
Run-TryBot: mzh <mzh@golangcn.org>
Reviewed-by: Joel Sing <joel@sing.id.au>
Reviewed-by: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>