New test gdb.arch/arm-neon.exp

Both ARM and AArch64 have defined some SIMD data types in arm_neon.h,
but we don't have a test case for passing them and returning them in
inferior call.  This test also covers passing and returning
homogeneous short vector aggregate (defined by AArch64 ABI document)
in inferior call too.

gdb/testsuite:

	* gdb.arch/arm-neon.exp: New.
	* gdb.arch/arm-neon.c: New.