[flang][Driver] When linking with the Fortran runtime also link with libexecinfo

Also link with libexecinfo on FreeBSD, NetBSD, OpenBSD and DragonFly
for the backtrace functions.

Author: brad0

clang/lib/Driver/ToolChains/CommonArgs.cpp

@@ -1340,6 +1340,11 @@ void tools::addFortranRuntimeLibs(const ToolChain &TC, const ArgList &Args,
     CmdArgs.push_back("-lFortranRuntime");
     CmdArgs.push_back("-lFortranDecimal");
     addArchSpecificRPath(TC, Args, CmdArgs);
+
+    // needs libexecinfo for backtrace functions
+    if (TC.getTriple().isOSFreeBSD() || TC.getTriple().isOSNetBSD() ||
+        TC.getTriple().isOSOpenBSD() || TC.getTriple().isOSDragonFly())
+      CmdArgs.push_back("-lexecinfo");
   }
 
   // libomp needs libatomic for atomic operations if using libgcc

flang/test/Driver/backtrace.f90

@@ -0,0 +1,3 @@
+!RUN: %flang --target=aarch64-unknown-openbsd -### %s 2>&1 | FileCheck --check-prefixes=BACKTRACE %s
+
+!BACKTRACE: -lexecinfo