JitArm64: Check fastmem instead of fastmem_arena

Preparation for the next commit.

JitArm64 has been conflating these two flags. Most of the stuff that's
been guarded by fastmem_arena checks in fact requires fastmem.

When we have fastmem_arena without fastmem, it would be possible to do
things a bit more efficiently than what this commit does, but it's
non-trivial and therefore I will leave it out of this PR. With this
commit, we effectively have the same behavior as before this PR - plus
the added ability to toggle fastmem with a cache clear.

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