Implement user fault handling with userfaultfd on Linux.

This commit implements the `uffd` feature which turns on support for utilizing
the `userfaultfd` system call on Linux for the pooling instance allocator.

By handling page faults in userland, we are able to detect guard page accesses
without having to constantly change memory page protections.

This should help reduce the number of syscalls as well as kernel lock
contentions when many threads are allocating and deallocating instances.

Additionally, the user fault handler can lazy initialize linear
memories of an instance (implementation to come).

Cargo.toml

@@ -89,6 +89,7 @@ jitdump = ["wasmtime/jitdump"]
 vtune = ["wasmtime/vtune"]
 wasi-crypto = ["wasmtime-wasi-crypto"]
 wasi-nn = ["wasmtime-wasi-nn"]
+uffd = ["wasmtime/uffd"]
 
 # Try the experimental, work-in-progress new x86_64 backend. This is not stable
 # as of June 2020.