Auto merge of #38079 - BurntSushi:attrtarget, r=alexcrichton

Add new #[target_feature = "..."] attribute.

This commit adds a new attribute that instructs the compiler to emit
target specific code for a single function. For example, the following
function is permitted to use instructions that are part of SSE 4.2:

    #[target_feature = "+sse4.2"]
    fn foo() { ... }

In particular, use of this attribute does not require setting the
-C target-feature or -C target-cpu options on rustc.

This attribute does not have any protections built into it. For example,
nothing stops one from calling the above `foo` function on hosts without
SSE 4.2 support. Doing so may result in a SIGILL.

I've also expanded the x86 target feature whitelist.

src/librustc_llvm/lib.rs

@@ -66,13 +66,13 @@ impl LLVMRustResult {
 
 pub fn AddFunctionAttrStringValue(llfn: ValueRef,
                                   idx: AttributePlace,
-                                  attr: &'static str,
-                                  value: &'static str) {
+                                  attr: &CStr,
+                                  value: &CStr) {
     unsafe {
         LLVMRustAddFunctionAttrStringValue(llfn,
                                            idx.as_uint(),
-                                           attr.as_ptr() as *const _,
-                                           value.as_ptr() as *const _)
+                                           attr.as_ptr(),
+                                           value.as_ptr())
     }
 }