Fix 1bit mutex test on platforms with 32 bit aligned pointers

Fix 1bit mutex test on platforms with 32 bit aligned pointers

The 1 bit mutex tests asserts: ((gsize) ptrs) % 8, ==, 0), which fails
when the platform only aligns porters to 32 bits (e.g. S390 and
powerpc).

I'm not sure why this assertion was placed here, but given
that internally g_pointer_bit_trylock uses g_atomic_int_or internally
change the assertion so it only requires the alignment to be a multiple
of sizeof(int)