Fix ldso name for for musl on x86 and x32 architectures

[raj.khem]

Summary

x32 linker is called ld-musl-x32.so.1 and x86 linker
is called ld-musl-i386.so.1, Currently, linker for
x86 is returned as ld-musl-i586.so.1, when default
arch is i586, which is not the right thing

[raj.khem]

This version works across all compilers.

[MaskRay]

This should need tests. I pushed f80471bb3e3c7a6f1225ff78de25a3efc3309902 for the i386 fix. The x32 change needs a new environment muslx32. See my patch CCing you:)

lib/Driver/ToolChains/Linux.cpp

   if (Triple.isMusl()) {
     std::string ArchName;
     bool IsArm = false;
+    bool isX32 = false;
 
     switch (Arch) {
     case llvm::Triple::arm:

       ArchName = "armeb";
       IsArm = true;
       break;
+    case llvm::Triple::x86:
+      ArchName = "i386";
+      break;
+    case llvm::Triple::x86_64:
+      isX32 = Triple.getEnvironment() == llvm::Triple::GNUX32;
+      ArchName = isX32 ? "x32" : Triple.getArchName().str();
+      break;
     default:
       ArchName = Triple.getArchName().str();
     }