[native_toolchain_c] Compile with -O3 by default (#1747)

[Not all linkers support `-Os`](https://logs.chromium.org/logs/dart/buildbucket/cr-buildbucket/8730301908709004769/+/u/test_results/new_test_failures__logs_), so defaulting to `-O3` is more portable.

Issue:

* #1267

pkgs/native_toolchain_c/CHANGELOG.md

@@ -2,7 +2,7 @@
 
 - For Android, produce dylibs with page-size set to 16kb by default.
   https://github.com/dart-lang/native/issues/1611
-- Make optimization level configurable. Defaults to `-Os` and `/Os`.
+- Make optimization level configurable. Defaults to `-3s` and `/O3`.
   https://github.com/dart-lang/native/issues/1267
 
 ## 0.6.0

pkgs/native_toolchain_c/lib/src/cbuilder/cbuilder.dart

@@ -68,7 +68,7 @@ class CBuilder extends CTool implements Builder {
     super.language = Language.c,
     super.cppLinkStdLib,
     super.linkModePreference,
-    super.optimizationLevel = OptimizationLevel.oS,
+    super.optimizationLevel = OptimizationLevel.o3,
   }) : super(type: OutputType.library);
 
   CBuilder.executable({
@@ -89,7 +89,7 @@ class CBuilder extends CTool implements Builder {
     super.std,
     super.language = Language.c,
     super.cppLinkStdLib,
-    super.optimizationLevel = OptimizationLevel.oS,
+    super.optimizationLevel = OptimizationLevel.o3,
   }) : super(
           type: OutputType.executable,
           assetName: null,

pkgs/native_toolchain_c/lib/src/cbuilder/clinker.dart

@@ -37,7 +37,7 @@ class CLinker extends CTool implements Linker {
     super.language = Language.c,
     super.cppLinkStdLib,
     super.linkModePreference,
-    super.optimizationLevel = OptimizationLevel.oS,
+    super.optimizationLevel = OptimizationLevel.o3,
   }) : super(type: OutputType.library);
 
   /// Runs the C Linker with on this C build spec.