[AArch64] Require aarch64_sve256_hw for a 256-bit SVE test

One of the SVE run tests was specific to 256-bit SVE but tried to
run for all SVE lengths.

2020-01-10  Richard Sandiford  <richard.sandiford@arm.com>

gcc/testsuite/
	* gcc.target/aarch64/sve/index_1_run.c: Require aarch64_sve256_hw
	rather than aarch64_sve_hw.

From-SVN: r280104