testsuite: Compile asan_test.C with -Wno-alloc-size-larger-than

testsuite: Compile asan_test.C with -Wno-alloc-size-larger-than

asan_test.cc tries to allocate 0xf0000000 bytes for 32bit targets in
a disabled DISABLED_DemoOOM test.  Since the testcase is compiled with
-Werror, the compilation fails with:

error: argument 1 value '4026531840' exceeds maximum object size 2147483647

Compile with -Wno-alloc-size-larger-than to avoid compilation failure.

	* g++.dg/asan/asan_test.C (dg-options): Add
	-Wno-alloc-size-larger-than.