build-llvm.py: Only apply native optimizations if specifically requested

This may cause issues with older versions of LLVM.

At the same time, drop the -O2 flag; release LLVM builds are compiled
with -O3 and it overrides our choice.

Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>

build-llvm.py

@@ -104,6 +104,14 @@ def parse_parameters(root_folder):
                         By default, the script always updates the LLVM repo before building. This prevents
                         that, which can be helpful during something like bisecting.
 
+                        """),
+                        action="store_true")
+    parser.add_argument("-o",
+                        "--opt-for-native",
+                        help=textwrap.dedent("""\
+                        Add -march=native and -mtune=native to CFLAGS to further optimize the toolchain for
+                        the host processor.
+
                         """),
                         action="store_true")
     parser.add_argument("-p",
@@ -503,11 +511,13 @@ def stage_specific_cmake_defines(args, dirs, stage):
     """
     defines = {}
 
+    if args.opt_for_native:
+        defines['CMAKE_C_FLAGS'] = '-march=native -mtune=native'
+        defines['CMAKE_CXX_FLAGS'] = '-march=native -mtune=native'
+
     if stage == 1 and not args.stage1_only:
         # Based on clang/cmake/caches/Apple-stage1.cmake
         defines['CMAKE_BUILD_TYPE'] = 'Release'
-        defines['CMAKE_C_FLAGS'] = '-O2 -march=native -mtune=native'
-        defines['CMAKE_CXX_FLAGS'] = '-O2 -march=native -mtune=native'
         defines['LLVM_ENABLE_BACKTRACES'] = 'OFF'
         defines['LLVM_ENABLE_WARNINGS'] = 'OFF'
         defines['LLVM_INCLUDE_TESTS'] = 'OFF'
@@ -516,14 +526,10 @@ def stage_specific_cmake_defines(args, dirs, stage):
         # If a debug build was requested
         if args.debug:
             defines['CMAKE_BUILD_TYPE'] = 'Debug'
-            defines['CMAKE_C_FLAGS'] = '-march=native -mtune=native'
-            defines['CMAKE_CXX_FLAGS'] = '-march=native -mtune=native'
             defines['LLVM_BUILD_TESTS'] = 'ON'
         # If a release build was requested
         else:
             defines['CMAKE_BUILD_TYPE'] = 'Release'
-            defines['CMAKE_C_FLAGS'] = '-O2 -march=native -mtune=native'
-            defines['CMAKE_CXX_FLAGS'] = '-O2 -march=native -mtune=native'
             defines['LLVM_ENABLE_WARNINGS'] = 'OFF'
             defines['LLVM_INCLUDE_TESTS'] = 'OFF'