[RISCV] Add tests for .attribute stack_align

As suggested by @asb [here](https://reviews.llvm.org/D143570#4112877), we need tests for `.attribute stack_align`. This patch simply verifies a stack alignment of 16 is emitted for any combination of currently supported base ISA and extensions.

Reviewed By: asb, kito-cheng

Differential Revision: https://reviews.llvm.org/D143639