Hard-coded page sizes

[hmelder]

On 64-bit POWER platforms, a page is 64 KB in size (at least on Linux by default). Similarly, Asahi Linux uses a 16 KB page size.

Currently, a page size of 4k is hard-coded in the block trampolines and block_to_imp.c. In alloc_trampolines we set PROT_EXEC for rx_buffer, but this assumes that rx_buffer points to the beginning of a 4k page.

There is one proper solution, and one duck tape fix:

1. Determine page size on runtime. Requires restructuring of block_to_imp.c as struct fields are determined based on PAGE_SIZE.
2. Determine page size during configuration