RISC-V: Give stack slots same align as XLEN.

RISC-V: Give stack slots same align as XLEN.

For riscv64-linux target, fixes
FAIL: gdb.base/gnu_vector.exp: call add_many_charvecs

Ensure that stack slots are always the same alignment as XLEN by rounding
up arg align to xlen.

	gdb/
	* riscv-tdep.c (riscv_call_arg_scalar_int): Use std::min when
	setting len.  New local align, set to max of arg align and xlen,
	and pass to first riscv_assign_stack_location call.