Enable PACBTI on OpenBSD/arm64. (#1792)

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 bootstrapping purposes, we need to make this change in the legacy C++
driver in swiftlang/swift#78394; this is the equivalent change for the
new driver.

It's not quite clear whether the flags need to be introduced in
BackendJob.swift as well as FrontendJobHelpers.swift; just in case, add
them in both locations.

Author: 3405691582

Sources/SwiftDriver/Toolchains/GenericUnixToolchain.swift

@@ -161,5 +161,15 @@ public final class GenericUnixToolchain: Toolchain {
         try commandLine.appendPath(VirtualPath(path: sysroot.pathString))
       }
     }
+
+
+    if driver.targetTriple.os == .openbsd && driver.targetTriple.arch == .aarch64 {
+      commandLine.appendFlag(.Xcc)
+      commandLine.appendFlag("-Xclang=-msign-return-address=non-leaf")
+      commandLine.appendFlag(.Xcc)
+      commandLine.appendFlag("-Xclang=-msign-return-address-key=a_key")
+      commandLine.appendFlag(.Xcc)
+      commandLine.appendFlag("-Xclang=-mbranch-target-enforce")
+    }
   }
 }