test: export isRiscv64 from common module

isRiscv64 is defined and used internally for the timeout multiplier
but was not exported. Export it alongside the other platform booleans
(isLinux, isFreeBSD, isPi, etc.) so tests can use common.isRiscv64
instead of inlining process.arch checks.

Refs: nodejs/build#4099
Signed-off-by: Jamie Magee <jamie.magee@gmail.com>

Author: JamieMagee

test/common/index.js

@@ -988,6 +988,7 @@ const common = {
   isOpenBSD,
   isMacOS,
   isPi,
+  isRiscv64,
   isSunOS,
   isWindows,
   localIPv6Hosts,