testsuite: Force -mcmodel=small for gcc.target/aarch64/pr115258.c

The test implicitly assumed the default code model and so failed
for -mcmodel=tiny.

gcc/testsuite/
	* gcc.target/aarch64/pr115258.c: Add -mcmodel=small.

(cherry picked from commit 3584aab37f54bcd220c7061568af777e37f4f6ed)
1 file changed