The following errors with 'unexpected token in argument list' with the carat under the first character of the value.
.ltorg
plusone DCFS 1.0plusone DCFS 0x7f800000
Why is this failing?
Probably because that's legacy ARM assembly syntax, but your current version of MDK is not using a legacy ARM assembler any more.
Use -masm=auto to invoke the legacy assembler:
https://developer.arm.com/documentation/101754/0621/armclang-Reference/armclang-Command-line-Options/-masm