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/rustllvm.h

@@ -97,6 +97,7 @@ enum LLVMRustAttribute {
   SanitizeThread = 20,
   SanitizeAddress = 21,
   SanitizeMemory = 22,
+  NonLazyBind = 23,
 };
 
 typedef struct OpaqueRustString *RustStringRef;