[llvm-jitlink] Teach InProcessDeltaMapper to honor -slab-page-size option.

The -slab-page-size option is used to set a simulated page size in -no-exec
tests. In order for this to work we need to use read/write permissions only
on all simulated pages in order to ensure that no simulated page is made
read-only by a permission change to the underlying real page.

The aim of this patch is to make it safe to enable ExecutionEngine regression
tests on arm64. Those tests will be enabled in a follow-up patch.