RISC-V: Add -mpreferred-stack-boundary option.

RISC-V: Add -mpreferred-stack-boundary option.

	2018-01-23  Andrew Waterman  <andrew@sifive.com>
	gcc/
	* config/riscv/riscv.c (riscv_stack_boundary): New.
	(riscv_option_override): Set riscv_stack_boundary.  Handle
	riscv_preferred_stack_boundary_arg.
	* config/riscv/riscv.h (MIN_STACK_BOUNDARY, ABI_STACK_BOUNDARY): New.
	(BIGGEST_ALIGNMENT): Set to STACK_BOUNDARY.
	(STACK_BOUNDARY): Set to riscv_stack_boundary.
	(RISCV_STACK_ALIGN): Use STACK_BOUNDARY.
	* config/riscv/riscv.opt (mpreferred-stack-boundary): New.
	* doc/invoke.tex (RISC-V Options): Add -mpreferred-stack-boundary.

Co-Authored-By: Jim Wilson <jimw@sifive.com>
From-SVN: r257005