Update memfd support with a runtime toggle (#3778)

This commit updates the `memfd` support in Wasmtime to have a runtime
toggle as to whether it's used or not. The compile-time feature gating
`memfd` support is now also re-enabled by default, but the new runtime
switch is still disabled-by-default.

Additionally this commit updates our fuzz oracle to turn on/off the
memfd flag to re-enable fuzzing with memfd on oss-fuzz.

Cargo.toml

@@ -96,6 +96,7 @@ default = [
   "wasmtime/parallel-compilation",
   "wasi-nn",
   "pooling-allocator",
+  "memfd",
 ]
 jitdump = ["wasmtime/jitdump"]
 vtune = ["wasmtime/vtune"]