Compile with -lpthread by default on linux.

This allows zlib_bench to link properly and we can use the "all" build
target again.

Change-Id: I95ae0c41fb988245dd472da81cb06424d1209786
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/117212
Reviewed-by: Nicholas Shahan <nshahan@google.com>
Commit-Queue: Mayank Patke <fishythefish@google.com>

build/config/linux/BUILD.gn

@@ -11,15 +11,13 @@ config("sdk") {
     "-lc++",
     "-lc",
     "-lm",
+    "-lpthread",
     "-lclang_rt.builtins",
     "-Wl,--exclude-libs=libc++.a",
   ]
 
   if (is_asan) {
-    ldflags += [
-      "-lpthread",
-      "-lrt",
-    ]
+    ldflags += [ "-lrt" ]
   }
 
   if (sysroot != "") {