aarch64: Update sizeless tests for recent GNU C changes

The tests for sizeless SVE types include checks that the types
are handled for initialisation purposes in the same way as scalars.
GNU C and C2x now allow scalars to be initialised using empty braces,
so this patch updates the SVE tests to match.

gcc/testsuite/
	* gcc.target/aarch64/sve/acle/general-c/gnu_vectors_1.c: Update
	tests for empty initializers.
	* gcc.target/aarch64/sve/acle/general-c/gnu_vectors_2.c: Likewise.
	* gcc.target/aarch64/sve/acle/general-c/sizeless-1.c: Likewise.
	* gcc.target/aarch64/sve/acle/general-c/sizeless-2.c: Likewise.