Guard aarch64/aapcs64 tests using abitest.S by check_weak_available

2019-12-16  Joel Brobecker  <brobecker@adacore.com>
           Olivier Hainque  <hainque@adacore.com>

	* gcc.target/aarch64/aapcs64/aapcs64.exp: Guard tests using
	abitest.S by check_weak_available.

Co-Authored-By: Olivier Hainque <hainque@adacore.com>

From-SVN: r279793