Use jemalloc-sys on Linux and OSX compilers

This commit adds opt-in support to the compiler to link to `jemalloc` in
the compiler. When activated the compiler will depend on `jemalloc-sys`,
instruct jemalloc to unprefix its symbols, and then link to it. The
feature is activated by default on Linux/OSX compilers for x86_64/i686
platforms, and it's not enabled anywhere else for now. We may be able to
opt-in other platforms in the future! Also note that the opt-in only
happens on CI, it's otherwise unconditionally turned off by default.

Closes #36963

src/bootstrap/lib.rs

@@ -530,7 +530,11 @@ impl Build {
 
     /// Get the space-separated set of activated features for the compiler.
     fn rustc_features(&self) -> String {
-        String::new()
+        let mut features = String::new();
+        if self.config.jemalloc {
+            features.push_str("jemalloc");
+        }
+        features
     }
 
     /// Component directory that Cargo will produce output into (e.g.