Adjust -Ctarget-cpu=native handling in cg_llvm

When cg_llvm encounters the `-Ctarget-cpu=native` it computes an
explciit set of features that applies to the target in order to
correctly compile code for the host CPU (because e.g. `skylake` alone is
not sufficient to tell if some of the instructions are available or
not).

However there were a couple of issues with how we did this. Firstly, the
order in which features were overriden wasn't quite right – conceptually
you'd expect `-Ctarget-cpu=native` option to override the features that
are implicitly set by the target definition. However due to how other
`-Ctarget-cpu` values are handled we must adopt the following order
of priority:

* Features from -Ctarget-cpu=*; are overriden by
* Features implied by --target; are overriden by
* Features from -Ctarget-feature; are overriden by
* function specific features.

Another problem was in that the function level `target-features`
attribute would overwrite the entire set of the globally enabled
features, rather than just the features the
`#[target_feature(enable/disable)]` specified. With something like
`-Ctarget-cpu=native` we'd end up in a situation wherein a function
without `#[target_feature(enable)]` annotation would have a broader
set of features compared to a function with one such attribute. This
turned out to be a cause of heavy run-time regressions in some code
using these function-level attributes in conjunction with
`-Ctarget-cpu=native`, for example.

With this PR rustc is more careful about specifying the entire set of
features for functions that use `#[target_feature(enable/disable)]` or
`#[instruction_set]` attributes.

Sadly testing the original reproducer for this behaviour is quite
impossible – we cannot rely on `-Ctarget-cpu=native` to be anything in
particular on developer or CI machines.

src/test/assembly/target-feature-multiple.rs

@@ -0,0 +1,41 @@
+// assembly-output: emit-asm
+// needs-llvm-components: x86
+// revisions: TWOFLAGS SINGLEFLAG
+// compile-flags: --target=x86_64-unknown-linux-gnu
+// [TWOFLAGS] compile-flags: -C target-feature=+rdrnd -C target-feature=+rdseed
+// [SINGLEFLAG] compile-flags: -C target-feature=+rdrnd,+rdseed
+
+// Target features set via flags aren't necessarily reflected in the IR, so the only way to test
+// them is to build code that requires the features to be enabled to work.
+//
+// In this particular test if `rdrnd,rdseed` somehow didn't make it to LLVM, the instruction
+// selection should crash.
+//
+// > LLVM ERROR: Cannot select: 0x7f00f400c010: i32,i32,ch = X86ISD::RDSEED 0x7f00f400bfa8:2
+// > In function: foo
+//
+// See also src/test/codegen/target-feature-overrides.rs
+#![feature(no_core, lang_items, link_llvm_intrinsics, abi_unadjusted)]
+#![crate_type = "lib"]
+#![no_core]
+
+#[lang = "sized"]
+trait Sized {}
+#[lang = "copy"]
+trait Copy {}
+
+// Use of these requires target features to be enabled
+extern "unadjusted" {
+    #[link_name = "llvm.x86.rdrand.32"]
+    fn x86_rdrand32_step() -> (u32, i32);
+    #[link_name = "llvm.x86.rdseed.32"]
+    fn x86_rdseed32_step() -> (u32, i32);
+}
+
+#[no_mangle]
+pub unsafe fn foo() -> (u32, u32) {
+    // CHECK-LABEL: foo:
+    // CHECK: rdrand
+    // CHECK: rdseed
+    (x86_rdrand32_step().0, x86_rdseed32_step().0)
+}