testsuite, aarch64: Check for non-default mcmodel settings

In some cases we want to be able to check whether a given mcmodel
value is accepted by the target (it is not sufficient just to set
-mcmodel=, that will cause the test to fail under those conditions
rather than being UNSUPPORTED).

gcc/testsuite/ChangeLog:

	* lib/target-supports.exp: Provide aarch64_mcmodel_large/tiny.
	* gcc.target/aarch64/pr70120-2.c: Require mcmodel support.
	* gcc.target/aarch64/pr78255.c: Likewise.
	* gcc.target/aarch64/pr78561.c: Likewise.
	* gcc.target/aarch64/pr78733.c: Likewise.
	* gcc.target/aarch64/pr79041-2.c: Likewise.
	* gcc.target/aarch64/pr94530.c: Likewise.
	* gcc.target/aarch64/reload-valid-spoff.c: Likewise.
	* gcc.target/aarch64/symbol-range-tiny.c: Likewise.

Signed-off-by: Iain Sandoe <iain@sandoe.co.uk>