Add link time optimization to clang/gcc builds

Using `-flto` on clang without using the gold linker produces the error:
```main.o: file not recognized: File format not recognized```
https://github.com/android/ndk/issues/242

`gold` seems to be faster than `ld`, and decently tested even in 2010:
https://stackoverflow.com/questions/3476093/replacing-ld-with-gold-any-experience

Qmake will still append `-Wl,-O1` even though higher optimization levels
are used. Removing this in order to keep the compile command as short as
possible, and to reduce weird issues down the line. Ref topic:
https://stackoverflow.com/questions/53384988/why-qmake-passes-wl-o1-to-gcc-when-linking-and-will-it-harm-lto

build_scripts/qt/compilers/clang-gcc-common-switches.pri

@@ -9,3 +9,8 @@ QMAKE_CXXFLAGS += -Wnon-virtual-dtor
 QMAKE_CXXFLAGS += -Wcast-align -Wunused -Woverloaded-virtual -Wformat=2
 
 QMAKE_CXXFLAGS += -Wdouble-promotion
+
+# Enable Link Time Optimization
+QMAKE_CXXFLAGS += -flto
+# Clang LTO requires the gold linker instead of ld
+QMAKE_LFLAGS += -flto -fuse-ld=gold

build_scripts/qt/compilers/gcc.pri

@@ -31,3 +31,6 @@ QMAKE_CXXFLAGS += -Wduplicated-branches -Wduplicated-cond -Wlogical-op -Wrestric
 QMAKE_CXXFLAGS += -Wconversion -Wno-sign-conversion
 
 QMAKE_CXXFLAGS += -Wzero-as-null-pointer-constant
+
+# Qmake will append this even though higher optimization levels are used.
+QMAKE_LFLAGS_RELEASE -= -Wl,-O1