The empty string in -T "" means that the linker script file location setting has something in it, but cannot be computed. When I exported a project from mbed to lpcxpresso, I found "${workspace_loc:/${ProjName}/None}" in managed linker settings, which points to nothing, and causes the error.
My export also did not have the toolchain object and linker files when exporting my project. I still haven't figure out where proper library files are and how to link them manually. I created a duplicate project that used the mbed_src library. So when I exported I have the entire source code and it compiled completely first try.