go/build, runtime/internal/sys: reserve RISC-V arch names

go/build, runtime/internal/sys: reserve RISC-V arch names

In #17528 it was discussed (off-topic to the actual issue) to reserve
GOARCH names for the RISC-V architecture. With the first RISC-V
Linux-capable development boards released (e.g. HiFive Unleashed),
Linux distributions being ported to RISC-V (e.g. Debian, Fedora) and
RISC-V support being added to gccgo (CL 96377), it becomes more likely
that Go software (and maybe Go itself) will be ported as well.

Add riscv and riscv64 (which is already used by gccgo), so Go 1.11 will
already recognize "*_riscv{,64}.go" as reserved files.

Change-Id: I042aab19c68751d82ea513e40f7b1d7e1ad924ea
Reviewed-on: https://go-review.googlesource.com/106256
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>