build-llvm: Enable LLD_DEFAULT_LD_LLD_IS_MINGW for builds on Windows

This makes sure we get the right behaviour (and output from "--help")
if ld.lld.exe is called directly without a wrapper setting the -m
parameter.

build-llvm.sh

@@ -222,6 +222,7 @@ if [ -n "$TARGET_WINDOWS" ]; then
     CMAKEFLAGS="$CMAKEFLAGS -DCLANG_DEFAULT_UNWINDLIB=libunwind"
     CMAKEFLAGS="$CMAKEFLAGS -DCLANG_DEFAULT_CXX_STDLIB=libc++"
     CMAKEFLAGS="$CMAKEFLAGS -DCLANG_DEFAULT_LINKER=lld"
+    CMAKEFLAGS="$CMAKEFLAGS -DLLD_DEFAULT_LD_LLD_IS_MINGW=ON"
 fi
 
 if [ -n "$LTO" ]; then