tests: mem_alloc: workaround aggressive optimization

As we don't use memory allocated in test_calloc, test_no_mem_malloc,
and test_no_mem_realloc. malloc call can be optimized away (that really
happens with ARC MWDT toolchain). That breaks the test. So disable
optimization for these functions.

Signed-off-by: Watson Zeng <zhiwei@synopsys.com>