Support for disabling the PLT on ELF targets

Disable the PLT where possible to improve performance
for indirect calls into shared libraries.

This optimization is enabled by default where possible.

- Add the `NonLazyBind` attribute to `rustllvm`:
  This attribute informs LLVM to skip PLT calls in codegen.

- Disable PLT unconditionally:
  Apply the `NonLazyBind` attribute on every function.

- Only enable no-plt when full relro is enabled:
  Ensures we only enable it when we have linker support.

- Add `-Z plt` as a compiler option

src/rustllvm/RustWrapper.cpp

@@ -178,6 +178,8 @@ static Attribute::AttrKind fromRust(LLVMRustAttribute Kind) {
     return Attribute::SanitizeAddress;
   case SanitizeMemory:
     return Attribute::SanitizeMemory;
+  case NonLazyBind:
+    return Attribute::NonLazyBind;
   }
   report_fatal_error("bad AttributeKind");
 }