By default, armclang -M creates makefile dependencies rules based on the source filename:

armclang --target=aarch64-arm-none-eabi -mcpu=cortex-a53 -M test.c 
test.o: test.c header.h

The -MT option renames the target of the makefile dependency rule:

armclang --target=aarch64-arm-none-eabi -mcpu=cortex-a53 -M test.c -MT foo  
foo: test.c header.h

The compiler executes only the preprocessor step of the compilation. By default, output is on the standard output stream.

If you specify multiple source files, the -MT option renames the target of all dependency rules:

armclang --target=aarch64-arm-none-eabi -mcpu=cortex-a53 -M test1.c test2.c -MT foo        
foo: test1.c header.h
foo: test2.c header.h

Specifying multiple -MT options creates multiple targets for each rule:

armclang --target=aarch64-arm-none-eabi -mcpu=cortex-a53 -M test1.c test2.c -MT foo -MT bar
foo bar: test1.c header.h
foo bar: test2.c header.h