Improve results on the rustc-perf benchmark suite

I only ran the debug benchmarks, as check should be identical and release will definitively be faster because of much less optimizations by cg_clif.

Except for some stress-tests the clean and baseline incremental results are quite positive (~10-60% improvement, often ~40%) For clean incremental the results are much worse (easily ~200%), as compiled object files are not stored in the incremental cache (#760) For patched incremental the results are very mixed. Sometimes the difference is just a little bit less than clean incremental, while in other cases it is up to ~70% faster than cg_llvm.

packed-simd failed due to a verifier error. Edit(2020-03-11): Opened #919. hyper-2 failed due to unsized locals not being implemented (used for impl FnOnce for Box<FnOnce>). Edit(2020-03-11): Fixed in #916. style-servo failed due to running out of disk space.

diff --git a/collector/src/bin/rustc-perf-collector/execute.rs b/collector/src/bin/rustc-perf-collector/execute.rs
index 9aa2cc48..4f577183 100644
--- a/collector/src/bin/rustc-perf-collector/execute.rs
+++ b/collector/src/bin/rustc-perf-collector/execute.rs
@@ -203,13 +203,19 @@ impl<'a> CargoProcess<'a> {
     fn run_rustc(&mut self) -> anyhow::Result<()> {
         loop {
             let mut cmd = self.base_command(self.cwd, "rustc");
+            cmd.env("RUSTFLAGS", "-Cpanic=abort \
+            -Zcodegen-backend=~/Documents/cg_clif/target/release/librustc_codegen_cranelift.so \
+            --sysroot ~/Documents/cg_clif/build_sysroot/sysroot");
+            cmd.arg("--target").arg("x86_64-unknown-linux-gnu");
             cmd.arg("-p").arg(self.get_pkgid(self.cwd));
             match self.build_kind {
                 BuildKind::Check => {
+                    return Ok(());
                     cmd.arg("--profile").arg("check");
                 }
                 BuildKind::Debug => {}
                 BuildKind::Opt => {
+                    return Ok(());
                     cmd.arg("--release");
                 }
             }

Start with the rustc-perf benchmark results and collector/src/bin/rustc-perf-collector/execute.rs, then reproduce the reported debug, clean incremental, baseline incremental, and patched incremental cases. Review the failures referenced for packed-simd, hyper-2, and style-servo, along with #760, #916, and #919. Done should mean a clearly defined benchmark improvement with reproducible results and resolved or documented failures.