RISC-V: Remove vxrm parameter for vsadd[u] and vssub[u]

Computation of `vsadd`, `vsaddu`, `vssub`, and `vssubu` do not need the
rounding mode, therefore the intrinsics of these instructions do not have
the parameter for rounding mode control.

gcc/ChangeLog:

	* config/riscv/riscv-vector-builtins-bases.cc: remove rounding mode of
	vsadd[u] and vssub[u].
	* config/riscv/vector.md: Ditto.

gcc/testsuite/ChangeLog:

	* g++.target/riscv/rvv/base/bug-12.C: Adapt testcase.
	* g++.target/riscv/rvv/base/bug-14.C: Ditto.
	* g++.target/riscv/rvv/base/bug-18.C: Ditto.
	* g++.target/riscv/rvv/base/bug-19.C: Ditto.
	* g++.target/riscv/rvv/base/bug-20.C: Ditto.
	* g++.target/riscv/rvv/base/bug-21.C: Ditto.
	* g++.target/riscv/rvv/base/bug-22.C: Ditto.
	* g++.target/riscv/rvv/base/bug-23.C: Ditto.
	* g++.target/riscv/rvv/base/bug-3.C: Ditto.
	* g++.target/riscv/rvv/base/bug-8.C: Ditto.
	* gcc.target/riscv/rvv/base/binop_vx_constraint-100.c: Ditto.
	* gcc.target/riscv/rvv/base/binop_vx_constraint-101.c: Ditto.
	* gcc.target/riscv/rvv/base/binop_vx_constraint-102.c: Ditto.
	* gcc.target/riscv/rvv/base/binop_vx_constraint-103.c: Ditto.
	* gcc.target/riscv/rvv/base/binop_vx_constraint-104.c: Ditto.
	* gcc.target/riscv/rvv/base/binop_vx_constraint-105.c: Ditto.
	* gcc.target/riscv/rvv/base/binop_vx_constraint-106.c: Ditto.
	* gcc.target/riscv/rvv/base/binop_vx_constraint-107.c: Ditto.
	* gcc.target/riscv/rvv/base/binop_vx_constraint-108.c: Ditto.
	* gcc.target/riscv/rvv/base/binop_vx_constraint-109.c: Ditto.
	* gcc.target/riscv/rvv/base/binop_vx_constraint-110.c: Ditto.
	* gcc.target/riscv/rvv/base/binop_vx_constraint-111.c: Ditto.
	* gcc.target/riscv/rvv/base/binop_vx_constraint-112.c: Ditto.
	* gcc.target/riscv/rvv/base/binop_vx_constraint-113.c: Ditto.
	* gcc.target/riscv/rvv/base/binop_vx_constraint-114.c: Ditto.
	* gcc.target/riscv/rvv/base/binop_vx_constraint-115.c: Ditto.
	* gcc.target/riscv/rvv/base/binop_vx_constraint-116.c: Ditto.
	* gcc.target/riscv/rvv/base/binop_vx_constraint-117.c: Ditto.
	* gcc.target/riscv/rvv/base/binop_vx_constraint-118.c: Ditto.
	* gcc.target/riscv/rvv/base/binop_vx_constraint-119.c: Ditto.
	* gcc.target/riscv/rvv/base/binop_vx_constraint-97.c: Ditto.
	* gcc.target/riscv/rvv/base/binop_vx_constraint-98.c: Ditto.
	* gcc.target/riscv/rvv/base/merge_constraint-1.c: Ditto.
	* gcc.target/riscv/rvv/base/fixed-point-vxrm-error.c: New test.
	* gcc.target/riscv/rvv/base/fixed-point-vxrm.c: New test.