Set LLVM_COMPILER_CHECKED when bootstrapping libunwind/libcxxabi/libcxx

The checks behind LLVM_COMPILER_CHECKED fail if there's no libc++
available for linking.

This fixes building libunwind/libcxxabi/libcxx with newer versions
of llvm, if the llvm installation isn't stripped.

build-libcxx.sh

@@ -99,6 +99,7 @@ build_all() {
             -DCMAKE_SYSTEM_NAME=Windows \
             -DCMAKE_C_COMPILER_WORKS=TRUE \
             -DCMAKE_CXX_COMPILER_WORKS=TRUE \
+            -DLLVM_COMPILER_CHECKED=TRUE \
             -DCMAKE_AR=$PREFIX/bin/llvm-ar \
             -DCMAKE_RANLIB=$PREFIX/bin/llvm-ranlib \
             -DLLVM_NO_OLD_LIBSTDCXX=TRUE \
@@ -148,6 +149,7 @@ build_all() {
             -DCMAKE_SYSTEM_NAME=Windows \
             -DCMAKE_C_COMPILER_WORKS=TRUE \
             -DCMAKE_CXX_COMPILER_WORKS=TRUE \
+            -DLLVM_COMPILER_CHECKED=TRUE \
             -DCMAKE_AR=$PREFIX/bin/llvm-ar \
             -DCMAKE_RANLIB=$PREFIX/bin/llvm-ranlib \
             -DLIBCXXABI_USE_COMPILER_RT=ON \
@@ -186,6 +188,7 @@ build_all() {
             -DCMAKE_SYSTEM_NAME=Windows \
             -DCMAKE_C_COMPILER_WORKS=TRUE \
             -DCMAKE_CXX_COMPILER_WORKS=TRUE \
+            -DLLVM_COMPILER_CHECKED=TRUE \
             -DCMAKE_AR=$PREFIX/bin/llvm-ar \
             -DCMAKE_RANLIB=$PREFIX/bin/llvm-ranlib \
             -DLIBCXX_USE_COMPILER_RT=ON \