sim: riscv: Make stack 16-byte aligned

Various gcc test cases fail due to the stack
alignment of 16 bytes is expected by gcc,
causing issues mostly with vararg functions,
e.g.

FAIL: gcc.c-torture/execute/nest-align-1.c   -O0  execution test
FAIL: gcc.c-torture/execute/nest-stdar-1.c   -O0  execution test
FAIL: gcc.c-torture/execute/va-arg-12.c   -O0  execution test
FAIL: gcc.c-torture/execute/va-arg-15.c   -O0  execution test
FAIL: gcc.c-torture/execute/va-arg-16.c   -O0  execution test
FAIL: gcc.c-torture/execute/va-arg-17.c   -O0  execution test
FAIL: gcc.c-torture/execute/va-arg-20.c   -O0  execution test
FAIL: gcc.c-torture/execute/va-arg-26.c   -O0  execution test
...

Approved-By: Andrew Burgess <aburgess@redhat.com>