make_armc5 exporter fails for online compiler

[bentcooke]

Using the NUCLEO_F411RE, export->make_armc5 fails with the following message.

[janjongboom]

Tracking here ARMmbed/mbed-os#6395.

[ciarmcom]

ARM Internal Ref: IOTCELL-773

[kivaisan]

Closing this issue as tracked with other issue.