RISC-V: Introduce -mvector-strict-align.

this patch disables movmisalign by default and introduces
the -mno-vector-strict-align option to override it and re-enable
movmisalign.  For now, generic-ooo is the only uarch that supports
misaligned vector access.

The patch also adds a check_effective_target_riscv_v_misalign_ok to
the testsuite which enables or disables the vector misalignment tests
depending on whether the target under test can execute a misaligned
vle32.

Changes from v3:
 - Adressed Kito's comments.
 - Made -mscalar-strict-align a real alias.

gcc/ChangeLog:

	* config/riscv/riscv-opts.h (TARGET_VECTOR_MISALIGN_SUPPORTED):
	Move from here...
	* config/riscv/riscv.h (TARGET_VECTOR_MISALIGN_SUPPORTED):
	...to here and map to riscv_vector_unaligned_access_p.
	* config/riscv/riscv.opt: Add -mvector-strict-align.
	* config/riscv/riscv.cc (struct riscv_tune_param): Add
	vector_unaligned_access.
	(riscv_override_options_internal): Set
	riscv_vector_unaligned_access_p.
	* doc/invoke.texi: Document -mvector-strict-align.

gcc/testsuite/ChangeLog:

	* lib/target-supports.exp: Add
	check_effective_target_riscv_v_misalign_ok.
	* gcc.dg/vect/costmodel/riscv/rvv/dynamic-lmul2-7.c: Add
	-mno-vector-strict-align.
	* gcc.dg/vect/costmodel/riscv/rvv/vla_vs_vls-10.c: Ditto.
	* gcc.dg/vect/costmodel/riscv/rvv/vla_vs_vls-11.c: Ditto.
	* gcc.dg/vect/costmodel/riscv/rvv/vla_vs_vls-12.c: Ditto.
	* gcc.dg/vect/costmodel/riscv/rvv/vla_vs_vls-8.c: Ditto.
	* gcc.dg/vect/costmodel/riscv/rvv/vla_vs_vls-9.c: Ditto.
	* gcc.target/riscv/rvv/autovec/vls/misalign-1.c: Ditto.