Manually add optimization flag for Android Release builds. (#12390)

With recent versions of NDK (since 23), the `-O` optimization level compile flag is not being passed when building in the "Release" configuration.
More details here: android/ndk#1740

Our "Release" Android builds have been built without the optimization flag since we upgraded from NDK 21.

This change is a workaround to manually add `-O3` for "Release" Android builds.

cmake/CMakeLists.txt

@@ -306,6 +306,16 @@ if (onnxruntime_EXTENDED_MINIMAL_BUILD AND NOT onnxruntime_MINIMAL_BUILD)
   set(onnxruntime_MINIMAL_BUILD ON)
 endif()
 
+# work around Android NDK bug which doesn't include -O flag for Release configuration
+# https://github.com/android/ndk/issues/1740
+# TODO: remove this when the NDK version(s) we support get fixed
+if (CMAKE_SYSTEM_NAME STREQUAL "Android")
+  # NB: attempting to match the effects of this fix: https://android-review.googlesource.com/c/platform/ndk/+/2168845
+  string(APPEND CMAKE_C_FLAGS_RELEASE " -O3")
+  string(APPEND CMAKE_CXX_FLAGS_RELEASE " -O3")
+  string(APPEND CMAKE_ASM_FLAGS_RELEASE " -O3")
+endif()
+
 # Enable space optimization for gcc/clang
 # Cannot use "-ffunction-sections -fdata-sections" if we enable bitcode (iOS)
 if (NOT MSVC AND NOT onnxruntime_ENABLE_BITCODE)