testsuite: aarch64: Fix failing vector structure tests on big-endian

testsuite: aarch64: Fix failing vector structure tests on big-endian

Recent refactoring of the arm_neon.h header enabled better code
generation for intrinsics that manipulate vector structures. New
tests were also added to verify the benefit of these changes. It now
transpires that the code generation improvements are observed only on
little-endian systems. This patch restricts the code generation tests
to little-endian targets.

gcc/testsuite/ChangeLog:

2021-08-04  Jonathan Wright  <jonathan.wright@arm.com>

	* gcc.target/aarch64/vector_structure_intrinsics.c: Restrict
	tests to little-endian targets.