Revert "build-libcxx: Build with -ffunction-sections -fdata-sections"

This reverts commit bd9e16f732.

It turns out this actually isn't needed; since switching to
building via the llvm-project/runtimes directory (in
7b1d05f545), the cmake build
pulls in HandleLLVMOptions.cmake, which checks for and adds
-ffunction-sections and -fdata-sections if supported. Therefore,
we don't need to specify it manually at all.

build-libcxx.sh

@@ -97,8 +97,6 @@ build_all() {
             -DCMAKE_C_COMPILER=$arch-w64-mingw32-clang \
             -DCMAKE_CXX_COMPILER=$arch-w64-mingw32-clang++ \
             -DCMAKE_CXX_COMPILER_TARGET=$arch-w64-windows-gnu \
-            -DCMAKE_C_FLAGS="-ffunction-sections -fdata-sections" \
-            -DCMAKE_CXX_FLAGS="-ffunction-sections -fdata-sections" \
             -DCMAKE_CROSSCOMPILING=TRUE \
             -DCMAKE_SYSTEM_NAME=Windows \
             -DCMAKE_C_COMPILER_WORKS=TRUE \