Remove dependency on the region crate (#4407)

This commit removes Wasmtime's dependency on the `region` crate. The
motivation for this came about when I was updating dependencies and saw
that `region` had a new major version at 3.0.0 as opposed to our
currently used 2.3 track. In reviewing the use cases of `region` within
Wasmtime I found two trends in particular which motivated this commit:

* Some unix-specific areas of `wasmtime_runtime` use
  `rustix::mm::mprotect` instead of `region::protect` already. This
  means that the usage of `region::protect` for changing virtual memory
  protections was already inconsistent.

* Many uses of `region::protect` were already in unix-specific regions
  which could make use of `rustix`.

Overall I opted to remove the dependency on the `region` crate to avoid
chasing its versions over time. Unix-specific changes of protections
were easily changed to `rustix::mm::mprotect`. There were two locations
where a windows/unix split is now required and I subjectively ruled
"that seems ok". Finally removing `region` also meant that the "what is
the current page size" query needed to be inlined into
`wasmtime_runtime`, which I have also subjectively ruled "that seems
fine".

Finally one final refactoring here was that the `unix.rs` and `linux.rs`
split for the pooling allocator was merged. These two files already only
differed in one function so I slapped a `cfg_if!` in there to help
reduce the duplication.

Cargo.lock

@@ -3323,7 +3323,6 @@ dependencies = [
  "paste",
  "psm",
  "rayon",
- "region",
  "serde",
  "target-lexicon",
  "tempfile",
@@ -3568,7 +3567,6 @@ dependencies = [
  "ittapi-rs",
  "log",
  "object",
- "region",
  "rustc-demangle",
  "rustix",
  "serde",
@@ -3605,7 +3603,6 @@ dependencies = [
  "memoffset",
  "more-asserts",
  "rand 0.8.5",
- "region",
  "rustix",
  "thiserror",
  "wasmtime-environ",