[ELF2/Linkerscript] Implement parsing of OUTPUT_ARCH() command

[davide]

Summary

Needed by FreeBSD kernel linker script.

[ruiu]

LGTM

ELF/LinkerScript.cpp

private:
 
   void addFile(StringRef Path);
 
   void readAsNeeded();
   void readEntry();
   void readGroup();
   void readInclude();
   void readOutput();
+  void readOutputArch();
   void readOutputFormat();
   void readSearchDir();
 
   StringSaver Saver;
   std::vector<StringRef> Tokens;
   size_t Pos = 0;
 };
 }
 
 void LinkerScript::run() {
   while (!atEOF()) {
     StringRef Tok = next();
     if (Tok == "ENTRY") {
       readEntry();
     } else if (Tok == "GROUP" || Tok == "INPUT") {
       readGroup();
     } else if (Tok == "INCLUDE") {
       readInclude();
     } else if (Tok == "OUTPUT") {
       readOutput();
+    } else if (Tok == "OUTPUT_ARCH") {
+      readOutputArch();
     } else if (Tok == "OUTPUT_FORMAT") {
       readOutputFormat();
     } else if (Tok == "SEARCH_DIR") {
       readSearchDir();
     } else {
       error("unknown directive: " + Tok);
     }
   }
void LinkerScript::readOutput() {
   // -o <file> takes predecence over OUTPUT(<file>).
   expect("(");
   StringRef Tok = next();
   if (Config->OutputFile.empty())
     Config->OutputFile = Tok;
   expect(")");
 }
 
+void LinkerScript::readOutputArch() {
+  // Error checking only for now.
+  expect("(");
+  next();
+  expect(")");
+}
+
 void LinkerScript::readOutputFormat() {
   // Error checking only for now.
   expect("(");
   next();
   StringRef Tok = next();
   if (Tok == ")")
    return;
   if (Tok != ",")

test/elf2/linkerscript-outputarch.s

+# REQUIRES: x86
+# RUN: echo "OUTPUT_ARCH(x)" > %t.script
+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-freebsd %s -o %t1
+# RUN: ld.lld2 -shared -o %t2 %t1 %t.script
+# RUN: llvm-readobj %t2 > /dev/null
+
+# RUN: echo "OUTPUT_ARCH(x, y)" > %t.script
+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-freebsd %s -o %t1
+# RUN: not ld.lld2 -shared -o %t2 %t1 %t.script
+# RUN: llvm-readobj %t2 > /dev/null