MAP_FIXED will clobber any existing memory mapping, and is not safe
to use in a multi-threaded environment. Whether we like it or not,
we are a multi-threaded environment, because Qt initializes before
we get to main(), so it's already too late to safely use MAP_FIXED
by the time we get there.

Use MAP_FIXED_NOREPLACE instead. This is how MAP_FIXED should have
behaved from the beginning.

Obviously this means you'll need Linux 4.17+ and a semi-recent libc
to use PCSX2 now. But if you're running a 6 year old unsupported
kernel, you have bigger problems.

Fixes "random" startup crashes.