Propagate memory allocation failure to out_of_memory_NORETURN

Propagate memory allocation failure to out_of_memory_NORETURN

Provide the user with a hint of what caused an out of memory error.
And explain that some memory policies, like selinux deny_execmem
might cause Permission denied errors.

Add an err argument to out_of_memory_NORETURN. And change
am_shadow_alloc to return a SysRes (all three callers were already
checking for errors and calling out_of_memory_NORETURN).