makefile export: create .link_options.txt with echo
makefile export: create .link_options.txt with echo

$(file > $@.in, $(filter %.o, $^)) is not supported in GNU Make 3.81.
Create the linker response file with pipe redirect from echo command.
This is tested with Cygwin make and make 3.8.1 shipped with macOS.

Also, change the linker responce file name to .link_options.txt.
This is an internal file, not a build artifact.

andcor02/mbed-os,andcor02/mbed-os,mbedmicro/mbed,kjbracey-arm/mbed,mbedmicro/mbed,mbedmicro/mbed,kjbracey-arm/mbed,mbedmicro/mbed,andcor02/mbed-os,andcor02/mbed-os,kjbracey-arm/mbed,andcor02/mbed-os,mbedmicro/mbed,kjbracey-arm/mbed,andcor02/mbed-os