[driver][riscv] Support riscv64-linux-gnu multiarch paths

[aurel32]

Summary

This change adds support for the riscv64-linux-gnu multiarch:

• library path in getMultiarchTriple
• include path in AddClangSystemIncludeArgs

With this change LLVM is able to find libraries and include files on a debian riscv64 system.

[asb]

Many thanks for the patch. Could you please add some tests for this behaviour? I imagine you'll want to add a new directory in test/Driver/Inputs with a Debian tree skeleton. See D63497 for a relevant example.

Also, when submitting patches please try to generate the patch with -U99999 (see here) so we can see full context in the Phabricator Web UI.

clang/lib/Driver/ToolChains/Linux.cpp

     if (D.getVFS().exists(SysRoot + "/lib/powerpc64le-linux-gnu"))
       return "powerpc64le-linux-gnu";
     break;
+  case llvm::Triple::riscv64:
+    if (D.getVFS().exists(SysRoot + "/lib/riscv64-linux-gnu"))
+      return "riscv64-linux-gnu";
+    break;
   case llvm::Triple::sparc:
     if (D.getVFS().exists(SysRoot + "/lib/sparc-linux-gnu"))
       return "sparc-linux-gnu";

       "/usr/include/powerpc64-linux-gnu"};
   const StringRef PPC64LEMultiarchIncludeDirs[] = {
       "/usr/include/powerpc64le-linux-gnu"};
+  const StringRef RISCV64MultiarchIncludeDirs[] = {
+      "/usr/include/riscv64-linux-gnu"};
   const StringRef SparcMultiarchIncludeDirs[] = {
       "/usr/include/sparc-linux-gnu"};
   const StringRef Sparc64MultiarchIncludeDirs[] = {

   case llvm::Triple::ppc64le:
     MultiarchIncludeDirs = PPC64LEMultiarchIncludeDirs;
     break;
+  case llvm::Triple::riscv64:
+    MultiarchIncludeDirs = RISCV64MultiarchIncludeDirs;
+    break;
   case llvm::Triple::sparc:
     MultiarchIncludeDirs = SparcMultiarchIncludeDirs;
     break;