Enable PACBTI on OpenBSD/arm64. (#78394)

BTI enforcement is mandatory, which means if PAC and BTI instructions
are not emitted, then the compiled binary gets killed with SIGILL. The
platform default compiler achieves enabling PAC and BTI by embedding the
relevant enabled Clang compilation option flags into the local platform
toolchain, which affects C/C++ code generation.

For Swift however, to achieve the same effect, we would need to add the
relevant LLVM module flags in the IRGen process. But, since Swift uses
the Clang code generator when doing this, using the same option flag
approach will work here as well, and is probably preferable to
introducing operating system-dependent logic to the ClangImporter, for
example.

Finally, the stdlib needs to be built with PACBTI as well, since the
stdlib's global constructors get run when a compiled binary does. Since
the Swift build uses the just-built Clang for the stdlib, just embed the
necessary options into `CMAKE_C_FLAGS` and `CMAKE_CXX_FLAGS` via
`build-script-impl`. This will be redundant with the host compiler, but
at least it will be thorough.

Author: 3405691582

lib/Driver/ToolChains.cpp

@@ -204,6 +204,15 @@ void ToolChain::addCommonFrontendArgs(const OutputInfo &OI,
     arguments.push_back("-disable-objc-interop");
   }
 
+  if (Triple.isOSOpenBSD() && Triple.getArch() == llvm::Triple::aarch64) {
+    arguments.push_back("-Xcc");
+    arguments.push_back("-Xclang=-mbranch-target-enforce");
+    arguments.push_back("-Xcc");
+    arguments.push_back("-Xclang=-msign-return-address=non-leaf");
+    arguments.push_back("-Xcc");
+    arguments.push_back("-Xclang=-msign-return-address-key=a_key");
+  }
+
   if (inputArgs.getLastArg(options::OPT_experimental_serialize_debug_info)) {
     arguments.push_back(
         inputArgs.MakeArgString(Twine("-experimental-serialize-debug-info")));

utils/build-script-impl

@@ -1425,6 +1425,9 @@ function swift_c_flags() {
         linux-static-*)
             echo -n " -D_GNU_SOURCE -DHAVE_DECL_PROGRAM_INVOCATION_SHORT_NAME"
             ;;
+        openbsd-aarch64)
+            echo -n " -Xclang=-msign-return-address=non-leaf -Xclang=-msign-return-address-key=a_key -Xclang=-mbranch-target-enforce"
+            ;;
     esac
 }