Enable SafeStack for CloudABI.

[ed]

Summary

I've got a patchset in my home directory to integrate support for
SafeStack into CloudABI's C library. All of the CloudABI unit tests
still seem to pass. Pretty sweet!

This change adds the necessary changes to Clang to make
-fsanitize=safe-stack work on CloudABI. Without it, passing this command
line flag throws an error.

[samsonov]

Please add a test case for this (probably to test/Driver/fsanitize.c).

[ed]

Also add a testcase.

[ed]

In D17243#353661, @samsonov wrote:

Please add a test case for this (probably to test/Driver/fsanitize.c).

Done!

[samsonov]

LGTM

lib/Driver/ToolChains.h

public:
   void AddClangCXXStdlibIncludeArgs(
       const llvm::opt::ArgList &DriverArgs,
       llvm::opt::ArgStringList &CC1Args) const override;
   void AddCXXStdlibLibArgs(const llvm::opt::ArgList &Args,
                            llvm::opt::ArgStringList &CmdArgs) const override;
 
   bool isPIEDefault() const override { return false; }
 
+  SanitizerMask getSupportedSanitizers() const override;
+
 protected:
   Tool *buildLinker() const override;
 };
 
 class LLVM_LIBRARY_VISIBILITY Solaris : public Generic_GCC {
 public:
   Solaris(const Driver &D, const llvm::Triple &Triple,
           const llvm::opt::ArgList &Args);

lib/Driver/ToolChains.cpp

void CloudABI::AddCXXStdlibLibArgs(const ArgList &Args,
   CmdArgs.push_back("-lc++abi");
   CmdArgs.push_back("-lunwind");
 }
 
 Tool *CloudABI::buildLinker() const {
   return new tools::cloudabi::Linker(*this);
 }
 
+SanitizerMask CloudABI::getSupportedSanitizers() const {
+  SanitizerMask Res = ToolChain::getSupportedSanitizers();
+  Res |= SanitizerKind::SafeStack;
+  return Res;
+}
+
 /// OpenBSD - OpenBSD tool chain which can call as(1) and ld(1) directly.
 
 OpenBSD::OpenBSD(const Driver &D, const llvm::Triple &Triple,
                  const ArgList &Args)
     : Generic_ELF(D, Triple, Args) {
   getFilePaths().push_back(getDriver().Dir + "/../lib");
   getFilePaths().push_back("/usr/lib");
 }