On error mmap returns MAP_FAILED(-1) not null.
FreeBSD was checking the return correctly, Linux was not.
This was noticed by triad attempting to run Dolphin under valgrind and not getting a memory space under the 2GB limit(Because -1 wraps around on
unsigned obviously)
MemoryUtil.cpp was incorrectly using the old __x86_64__ define when it should be using _M_X86_64.
It was also using _ARCH_64 when it shouldn't have which was causing an errant PanicAlert to come up in my development.
Our defines were never clear between what meant 64bit or x86_64
This makes a clear cut between bitness and architecture.
This commit also has the side effect of bringing up aarch64 compiling support.
- remove unused variables
- reduce the scope where it makes sense
- correct limits (did you know that strcat()'s last parameter does not
include the \0 that is always added?)
- set some free()'d pointers to NULL
