[MC] Add Subtarget for MAsmParser call to emitCodeAlignment

The call to emitCodeAlignment was missing a STI which is required
after D45962.

emitCodeAlignment has a default parameter of 0 for MaxBytesToEmit.
Explicitly passing 0 here was interpreted as as nullptr for the STI.
This could possibly be avoided by taking STI as a const reference in
emitCodeAlignment.

Differential Revision: https://reviews.llvm.org/D109425