RISC-V: Improve portability of testcases

stdint.h will require having corresponding multi-lib existing, so using
stdint-gcc.h instead, also added a riscv_vector.h wrapper to
gcc.target/riscv/rvv/autovec/.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/autovec/partial/single_rgroup-1.h: Change
	stdint.h to stdint-gcc.h.
	* gcc.target/riscv/rvv/autovec/template-1.h: Ditto.
	* gcc.target/riscv/rvv/autovec/riscv_vector.h: New.