[PATCH] [RISC-V]Support -mcpu for Xuantie cpu

[PATCH] [RISC-V]Support -mcpu for Xuantie cpu

Support -mcpu=xt-c908, xt-c908v, xt-c910, xt-c910v2, xt-c920, xt-c920v2
for Xuantie series cpu.
ref:https://www.xrvm.cn/community/download?id=4224248662731067392

without fmv_cost, vector_unaligned_access, use_divmod_expansion, overlap_op_by_pieces, fill the tune info with generic ooo for further modification.

gcc/ChangeLog:

	* config/riscv/riscv-cores.def (RISCV_TUNE): Add xt-c908, xt-c908v,
	xt-c910, xt-c910v2, xt-c920, xt-c920v2.
	(RISCV_CORE): Add xt-c908, xt-c908v, xt-c910, xt-c910v2, xt-c920,
	xt-c920v2.
	* doc/invoke.texi: Add xt-c908, xt-c908v, xt-c910, xt-c910v2,
	xt-c920, xt-c920v2.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/mcpu-xt-c908.c: test -mcpu=xt-c908.
	* gcc.target/riscv/mcpu-xt-c910.c: test -mcpu=xt-c910.
	* gcc.target/riscv/mcpu-xt-c920v2.c: test -mcpu=xt-c920v2.
	* gcc.target/riscv/mcpu-xt-c908v.c: test -mcpu=xt-c908v.
	* gcc.target/riscv/mcpu-xt-c910v2.c: test -mcpu=xt-c910v2.
	* gcc.target/riscv/mcpu-xt-c920.c: test -mcpu=xt-c920.