RISC-V: Handle vector type alignment.

RISC-V: Handle vector type alignment.

For riscv64-linux target, first half of fix for
FAIL: gdb.base/gnu_vector.exp: call add_various_floatvecs

GCC gives vectors natural aligment based on total size, not element size,
bounded by the maximum supported type alignment.

	gdb/
	* riscv-tdep.c (BIGGEST_ALIGNMENT): New.
	(riscv_type_alignment) <TYPE_CODE_ARRAY>: If TYPE_VECTOR, return min
	of TYPE_LENGTH and BIGGEST_ALIGNMENT.