[pkg/dartdev] Allow cross compilation to Linux ARM/RISCV64.

Note that Linux ARM cross compilation is not currently supported
on Windows.

Follow-up CL to:

* https://dart-review.googlesource.com/c/sdk/+/437720
* https://dart-review.googlesource.com/c/recipes/+/437740

TEST=ci

Issue: #28617
Change-Id: Ia91c9fa44101f7b4202ab717fe1cba6a3470bf31
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/437760
Reviewed-by: Ivan Inozemtsev <[email protected]>
Commit-Queue: Tess Strickland <[email protected]>

Author: sstrickl

pkg/dartdev/lib/src/commands/compile.dart

@@ -467,7 +467,10 @@ class CompileNativeCommand extends CompileSubcommandCommand {
   static const String exeCmdName = 'exe';
   static const String aotSnapshotCmdName = 'aot-snapshot';
   static final supportedTargetPlatforms = <Target>{
+    // ARM cross-compilation is not supported on Windows currently.
+    if (!Platform.isWindows) Target.linuxArm,
     Target.linuxArm64,
+    Target.linuxRiscv64,
     Target.linuxX64
   };