RISC-V: Clean up testsuite for multi-lib testing [NFC]

- Drop unnecessary including for stdlib.h and math.h
- Drop assert.h / assert, use __builtin_abort instead.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/autovec/binop/shift-scalar-template.h:
	Use __builtin_abort instead of assert.
	* gcc.target/riscv/rvv/autovec/cond/cond_fmax-1.c: Drop math.h.
	* gcc.target/riscv/rvv/autovec/cond/cond_fmax-2.c: Ditto.
	* gcc.target/riscv/rvv/autovec/cond/cond_fmax-3.c: Ditto.
	* gcc.target/riscv/rvv/autovec/cond/cond_fmax-4.c: Ditto.
	* gcc.target/riscv/rvv/autovec/cond/cond_fmin-1.c: Ditto.
	* gcc.target/riscv/rvv/autovec/cond/cond_fmin-2.c: Ditto.
	* gcc.target/riscv/rvv/autovec/cond/cond_fmin-3.c: Ditto.
	* gcc.target/riscv/rvv/autovec/cond/cond_fmin-4.c: Ditto.
	* gcc.target/riscv/rvv/autovec/cond/cond_fmax_zvfh-1.c: Ditto.
	* gcc.target/riscv/rvv/autovec/cond/cond_fmax_zvfh-2.c: Ditto.
	* gcc.target/riscv/rvv/autovec/cond/cond_fmax_zvfh-3.c: Ditto.
	* gcc.target/riscv/rvv/autovec/cond/cond_fmax_zvfh-4.c: Ditto.
	* gcc.target/riscv/rvv/autovec/cond/cond_fmin_zvfh-1.c: Ditto.
	* gcc.target/riscv/rvv/autovec/cond/cond_fmin_zvfh-2.c: Ditto.
	* gcc.target/riscv/rvv/autovec/cond/cond_fmin_zvfh-3.c: Ditto.
	* gcc.target/riscv/rvv/autovec/cond/cond_fmin_zvfh-4.c: Ditto.
	* gcc.target/riscv/rvv/autovec/partial/single_rgroup-2.h: Use
	__builtin_abort instead of assert.
	* gcc.target/riscv/rvv/autovec/pr112694-1.c: Ditto.
	* gcc.target/riscv/rvv/autovec/partial/single_rgroup-3.h: Ditto.
	* gcc.target/riscv/rvv/autovec/unop/abs-template.h: Drop stdlib.h.
	* gcc.target/riscv/rvv/autovec/unop/vneg-template.h: Ditto.
	* gcc.target/riscv/rvv/autovec/unop/vnot-template.h: Ditto.