Make sure that your Arm Compiler version is up to date – I think support for this flag is relatively recent (6.18ish). I’m unable to reproduce this error with 6.20:
$ make CC=armclang CFLAGS=-fstack-usage
$ find build -name '*.su'
build/fvp/release/libfdt/fdt_wip.su
build/fvp/release/libfdt/fdt.su
build/fvp/release/libfdt/fdt_rw.su
...
The stack usage is dumped as expected, e.g. build/fvp/release/bl31/psci_on.su:
lib/psci/psci_on.c:59:psci_cpu_on_start 48 static
lib/psci/psci_on.c:163:psci_cpu_on_finish 48 static
Chris