[RISCV] Fix crash for section alignment with .option norvc

The existing code wasn't getting the subtarget info from the fragment,
so the current status of RVC would be ignored. This would cause a crash
for the new test case when the target then reported it couldn't write
the requested number of code alignment bytes.

Differential Revision: https://reviews.llvm.org/D122236

(cherry picked from commit d09d297c)