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.

crates/runtime/Cargo.toml

@@ -14,7 +14,6 @@ edition = "2021"
 wasmtime-environ = { path = "../environ", version = "=0.40.0" }
 wasmtime-fiber = { path = "../fiber", version = "=0.40.0", optional = true }
 wasmtime-jit-debug = { path = "../jit-debug", version = "=0.40.0", features = ["gdb_jit_int"] }
-region = "2.1.0"
 libc = { version = "0.2.112", default-features = false }
 log = "0.4.8"
 memoffset = "0.6.0"
@@ -39,6 +38,7 @@ features = [
   "Win32_System_Kernel",
   "Win32_System_Memory",
   "Win32_System_Diagnostics_Debug",
+  "Win32_System_SystemInformation",
   "Win32_Storage_FileSystem",
   "Win32_Security",
 ]