reflect: correct riscv64 assembly

R0 is not a thing in riscv64 assembly - use ZERO (rather than X0) since
the rest of this currently uses ABI names.

Updates #27532

Change-Id: I28fb68e9f80d05231a07c5921e7062777234e2c5
Reviewed-on: https://go-review.googlesource.com/c/go/+/215437


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>