Fix Android -ltcg build

We're passing -Oz for release builds, but that's not a flag the linker
understands when -ltcg is enabled.  The build fails with:
    ld.gold: fatal error: Optimization level must be between 0 and 3

Fix this by using -O2, which -Oz is based on, and -O3 for the "full
optimization" that is used for core and gui.

Pick-to: 6.2
Fixes: QTBUG-89472
Change-Id: Ie1a86888baefce5ca97026e7d635f10d2819f9f4
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>

cmake/QtCompilerOptimization.cmake

@@ -168,8 +168,14 @@ endif()
 
 # Android Clang
 if(CLANG AND ANDROID)
-    set(QT_CFLAGS_OPTIMIZE "-Oz")
-    set(QT_CFLAGS_OPTIMIZE_FULL "-Oz")
+    if(QT_FEATURE_ltcg)
+        # When using LTCG, the linker cannot cope with -Oz.  See QTBUG-89472 for details.
+        set(QT_CFLAGS_OPTIMIZE "-O2")
+        set(QT_CFLAGS_OPTIMIZE_FULL "-O3")
+    else()
+        set(QT_CFLAGS_OPTIMIZE "-Oz")
+        set(QT_CFLAGS_OPTIMIZE_FULL "-Oz")
+    endif()
 endif()
 
 # qcc

configure.cmake

@@ -624,6 +624,12 @@ qt_feature("ltcg"
     CONDITION __qt_ltcg_detected
 )
 qt_feature_config("ltcg" QMAKE_PRIVATE_CONFIG)
+
+if(NOT QT_CONFIGURE_RUNNING)
+    # This feature is used early in QtCompilerOptimization.cmake.
+    qt_evaluate_feature(ltcg)
+endif()
+
 qt_feature("enable_new_dtags"
     LABEL "Using new DTAGS"
     CONDITION LINUX AND TEST_enable_new_dtags