testsuite: Expect a warning on aarch64 for declare-simd-coarray-lib.f90 [PR93660]

testsuite: Expect a warning on aarch64 for declare-simd-coarray-lib.f90 [PR93660]

aarch64 currently doesn't support declare simd where the return value and arguments
have different sizes and warns about that case.  This change adds a dg-warning
for that case like various other tests have already.

2021-03-29  Jakub Jelinek  <jakub@redhat.com>

	PR fortran/93660
	* gfortran.dg/gomp/declare-simd-coarray-lib.f90: Expect a mixed size
	declare simd warning on aarch64.