Correctly print RISC-V arch name on error when verifying ELF header.

Test: enable debug logging, observe EM_RISCV printed in dlopen error
message instead of default EM_???.

Change-Id: Ieed5bd3eecc5d4093ffcb40558c554bb747e7a4b

linker/linker_phdr.cpp

@@ -216,6 +216,7 @@ static const char* EM_to_string(int em) {
   if (em == EM_386) return "EM_386";
   if (em == EM_AARCH64) return "EM_AARCH64";
   if (em == EM_ARM) return "EM_ARM";
+  if (em == EM_RISCV) return "EM_RISCV";
   if (em == EM_X86_64) return "EM_X86_64";
   return "EM_???";
 }