Add minimal riscv64 elf support to allow create_minidebuginfo on riscv64 binaries

This adds support for riscv64 elf files so that create_minidebuginfo
can be used strip riscv64 binaries.

Test: m create_minidebuginfo
Change-Id: I0cb0348d46ffb216af2f5c61701406504fae3b22
diff --git a/compiler/debug/elf_debug_line_writer.h b/compiler/debug/elf_debug_line_writer.h
index 8d62747..7df301a 100644
--- a/compiler/debug/elf_debug_line_writer.h
+++ b/compiler/debug/elf_debug_line_writer.h
@@ -73,6 +73,7 @@
         code_factor_bits_ = 2;  // 32-bit instructions
         break;
       case InstructionSet::kNone:
+      case InstructionSet::kRiscv64:
       case InstructionSet::kX86:
       case InstructionSet::kX86_64:
         break;