Jit feature preparation: Introduce "proxy blocks".

When these are invalidated, the block they point to gets invalidated too.

Will be useful to implement various types of block merging and function inlining
without affecting correctness of cache clears etc.

Also, with this commit we can now fully inline replaced functions. fabsf() boils
down to 1-2 instructions and the block continues, for example.
A few more known funcs