Clang has a completely inconsistent CLI for its integrated assembler for
each target architecture. For x86_64, for example, it does not accept
an -mcpu parameter, and emits "warning: unused parameter". However, for
ARM, -mcpu is needed in order to properly lower assembly to machine code
instructions (see new standalone test case provided thanks to @g-w1).
This is a compromise between
b8f85a805b and
afb9f695b1.
