clang: Add -lpthread and -ldl along with -lunwind for static linking

When doing static liking with --unwindlib=libunwind -static we encounter
undefined symbols
libunwind/src/RWMutex.hpp:68: undefined reference to `pthread_rwlock_wrlock'

and

libunwind/src/AddressSpace.hpp:597: undefined reference to `dladdr'

therefore we need to link in libpthread and libdl to fill these symbols

Upstream-Status: Pending
Signed-off-by: Khem Raj <raj.khem@gmail.com>

Author: kraj

clang/lib/Driver/ToolChains/CommonArgs.cpp

@@ -2297,6 +2297,8 @@ static void AddUnwindLibrary(const ToolChain &TC, const Driver &D,
         CmdArgs.push_back("-lunwind");
     } else if (LGT == LibGccType::StaticLibGcc) {
       CmdArgs.push_back("-l:libunwind.a");
+      CmdArgs.push_back("-lpthread");
+      CmdArgs.push_back("-ldl");
     } else if (LGT == LibGccType::SharedLibGcc) {
       if (TC.getTriple().isOSCygMing())
         CmdArgs.push_back("-l:libunwind.dll.a");