cmd/vendor: cherry pick asm_linux_riscv64.s for x/sys/unix

Manually cherry pick asm_linux_riscv64.s from x/sys/unix rev 4c3a928,
as needed for the linux/riscv64 port.

Normally this would be a vendor update, however this is significantly
more risky at this point in time - a full update can be done post release.

Updates #27532

Change-Id: I03bb191bc44dcd24b9a29243957b88d7a9091852
Reviewed-on: https://go-review.googlesource.com/c/go/+/216260
Reviewed-by: Brad Fitzpatrick <[email protected]>

Author: 4a6f656c