[native_toolchain_c] Setup Android RISCV64 toolchain. (#165)

Author: rmacnak-google

pkgs/native_toolchain_c/lib/src/cbuilder/run_cbuilder.dart

@@ -133,7 +133,8 @@ class RunCBuilder {
     // See https://github.com/dart-lang/native/issues/171.
     late final int targetAndroidNdkApi;
     if (target.os == OS.android) {
-      targetAndroidNdkApi = max(buildConfig.targetAndroidNdkApi!, 21);
+      final minimumApi = target == Target.androidRiscv64 ? 35 : 21;
+      targetAndroidNdkApi = max(buildConfig.targetAndroidNdkApi!, minimumApi);
     }
 
     await runProcess(
@@ -294,6 +295,7 @@ class RunCBuilder {
     Target.androidArm64: 'aarch64-linux-android',
     Target.androidIA32: 'i686-linux-android',
     Target.androidX64: 'x86_64-linux-android',
+    Target.androidRiscv64: 'riscv64-linux-android',
   };
 
   static const appleClangMacosTargetFlags = {

pkgs/native_toolchain_c/test/cbuilder/cbuilder_cross_android_test.dart

@@ -17,20 +17,24 @@ void main() {
     Target.androidArm64,
     Target.androidIA32,
     Target.androidX64,
+    // TODO(rmacnak): Enable when stable NDK 27 is available.
+    // Target.androidRiscv64,
   ];
 
   const readElfMachine = {
     Target.androidArm: 'ARM',
     Target.androidArm64: 'AArch64',
     Target.androidIA32: 'Intel 80386',
     Target.androidX64: 'Advanced Micro Devices X86-64',
+    Target.androidRiscv64: 'RISC-V',
   };
 
   const objdumpFileFormat = {
     Target.androidArm: 'elf32-littlearm',
     Target.androidArm64: 'elf64-littleaarch64',
     Target.androidIA32: 'elf32-i386',
     Target.androidX64: 'elf64-x86-64',
+    Target.androidRiscv64: 'elf64-littleriscv',
   };
 
   /// From https://docs.flutter.dev/reference/supported-platforms.